u.md

U

UA : → User agent

UA string : → User agent string

UAAG : → User Agent Accessibility Guidelines

UAT : → User acceptance testing

UCD : → User-Centered Design

UDD : → User-Driven Development

UDJO : → Using Declarations Just Once

UDP : → User Datagram Protocol

UDP flood attack : A volumetric denial of service (DoS) attack using the User Datagram Protocol (UDP). A UDP flood attack can be initiated by sending a large number of UDP packets to random ports on a remote host. As a result, the distant host will check for the application listening at that port, find no application listening, and reply with an ICMP “Destination Unreachable” message. For a large number of UDP packets, the victimized system will be forced into sending many ICMP messages, eventually leading it to be unreachable by other clients. †

Uglification : → Minification : → Obfuscation

UI : → User Interface

UI framework : → Framework

UI redressing : → Clickjacking

UIML : → User interface markup language

UMD : → Universal Module Definition

UML : → Unified Modeling Language

undefined : A primitive value automatically assigned to variables that have just been declared, or to formal arguments for which there are no actual arguments. ‡

Underscore hack : → Star hack

Unicode : A computing industry standard for the consistent encoding, representation, and handling of text expressed in most of the world’s writing systems. The standard is maintained by the Unicode Consortium. Unicode 14.0.0 contains a repertoire of 144,697 characters covering 159 modern and historic scripts, as well as multiple symbol sets and emoji. The character repertoire of the Unicode Standard is synchronized with ISO/IEC 10646, and both are code-for-code identical. † ℹ︎ unicode.org/standard

Unicode Consortium : A non-profit organization whose primary purpose is to maintain and publish the Unicode Standard which was developed with the intention of replacing existing character encoding schemes which are limited in size and scope, and are incompatible with multilingual environments. The Consortium describes its overall purpose “to enable people around the world to use computers in any language, by providing freely available specifications and data to form the foundation for software internationalization in all major operating systems, search engines, applications, and the World Wide Web.” Unicode’s success at unifying character sets has led to its widespread adoption in the internationalization and localization of software. The Unicode Consortium was founded in 1991. † ℹ︎ unicode.org

Unicode Standard : → Unicode

Unified Modeling Language : A general-purpose, developmental, modeling language in the field of software engineering that is intended to provide a standard way to visualize the design of a system. The creation of UML was originally motivated by the desire to standardize the disparate notational systems and approaches to software design. It was developed in 1994 and 1995 by Grady Booch, Ivar Jacobson, and James Rumbaugh. † ℹ︎ uml.org

Uniform Resource Characteristic : A string of characters representing the metadata of a Uniform Resource Identifier (URI), a string identifying a web resource. A URC binds a URI’s associated Uniform Resource Name (URN), a unique name for a web resource, to its Uniform Resource Locator (URL), the location at which a web resource can be found. URCs were proposed as a specification in the mid-1990s, but were never adopted. †

Uniform Resource Identifier : A string of characters that unambiguously identifies a particular resource. To guarantee uniformity, all URIs follow a predefined set of syntax rules, but also maintain extensibility through a separately defined hierarchical naming scheme (e.g., “https://”). An identification as with URIs enables interaction with representations of the resource over a network, typically the World Wide Web, using specific protocols. Schemes specifying a concrete syntax and associated protocols define each URI. The most common form of URI is the Uniform Resource Locator (URL), frequently referred to informally as a web address. More rarely seen in usage is the Uniform Resource Name (URN). †

Uniform Resource Locator : A reference to a web resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identifier (URI), although many people use the two terms interchangeably. URLs occur most commonly to reference web pages (“http”), but are also used for file transfer (“ftp”), email (“mailto”), database access (JDBC), and many other applications. Most web browsers display the URL of a web page above the page in an address bar. A typical URL could have the form “https://www.example.com/contact.html”, which indicates a protocol (“https”), a hostname (“www.example.com”), and a filename (“contact.html”). Uniform Resource Locators were defined in RFC 1738 in 1994 by Tim Berners-Lee, the inventor of the World Wide Web, and the URI Working Group of the Internet Engineering Task Force (IETF). †

Uniform Resource Name : A Uniform Resource Identifier (URI) that uses the “urn” scheme. URNs were originally conceived to be part of a three-part information architecture for the Internet, along with Uniform Resource Locators (URLs) and Uniform Resource Characteristics (URCs), a metadata framework. URNs were conceived as persistent, location-independent identifiers assigned within defined namespaces. Since RFC 3986 in 2005, use of the terms “Uniform Resource Name” and “Uniform Resource Locator” has been deprecated in technical standards in favor of the term “Uniform Resource Identifier” (URI), which encompasses both. †

Uninitialized pointer : → Uninitialized variable

Uninitialized variable : A variable that is declared but is not set to a definite known value before it is used. It will have some value, but not a predictable one. As such, an uninitialized variable is a programming error and a common source of bugs in software. †

Unique Selling Point : The unique benefit exhibited by a product, service, brand, or company that enables it to stand out from competitors. The USP must be a feature that highlights product benefits that are meaningful to consumers, and it focuses on explicit claims of uniqueness involving an objectively verifiable product attribute or benefit in use. †

Unique Selling Proposition : → Unique Selling Point

Unique Value Proposition : → Unique Selling Point

Unit testing : A software testing method by which individual units of source code, sets of one or more computer program modules together with associated control data, usage procedures, and operating procedures, are tested to determine whether they are fit for use. Unit tests are typically automated tests written and run to ensure that a section of an application (known as the “unit”) meets its design and behaves as intended. In procedural programming, a unit could be an entire module, but it is more commonly an individual function or procedure. In object-oriented programming, a unit is often an entire interface, such as a class, but could be an individual method. †

Universal Access : A component of the Mac OS X operating system, later called Accessibility, that provides computing abilities to people with visual impairment, hearing impairment, or physical disability. † ℹ︎ apple.com/accessibility/mac

Universal Cross-Site Scripting : An attack that exploits vulnerabilities in a browser or browser extension (instead of a website or application, as with XSS) in order to run malicious code.

Universal Design : The design of buildings, products, or environments to make them accessible to all people, regardless of age, disability or other factors. The term “universal design” was coined by the architect Ronald Mace to describe the concept of designing all products and the built environment to be aesthetic and usable to the greatest extent possible by everyone, regardless of their age, ability, or status in life. However, it was the work of Selwyn Goldsmith, author of Designing for the Disabled (1963), who pioneered the concept of free access for people with disabilities. Goldsmith’s most significant achievement was the creation of the dropped curb—now a standard feature of the built environment. †

Universal Module Definition : A collection of patterns and scripts to help use and compatibility of different JavaScript module loaders. ℹ︎ github.com/umdjs/umd

Universal selector : A CSS selector (*, which can in certain cases be omitted) that matches any element.

Universally unique identifier : A 128-bit number used to identify information in computer systems. The term globally unique identifier (GUID) is also used, typically in software created by Microsoft. When generated according to the standard methods, UUIDs are for practical purposes unique. Their uniqueness does not depend on a central registration authority or coordination between the parties generating them, unlike most other numbering schemes. While the probability that a UUID will be duplicated is not zero, it is close enough to zero to be negligible. Thus, anyone can create a UUID and use it to identify something with near certainty that the identifier does not duplicate one that has already been, or will be, created to identify something else. †

Universal XSS : → Universal Cross-Site Scripting

Uniwidth typeface : A typeface where every variation (font) has the same metrics (size of each letter). As a result, changing the variation used, such as using bold or italics, does not change the layout (reflow). †

Unix/UNIX : A family of multi-tasking, multi-user computer operating systems that derive from the original AT&T Unix, with development starting in the 1970s at Bell Labs. Initially intended for use inside the Bell System, AT&T licensed Unix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants. Unix systems are characterized by a modular design that is sometimes called the Unix philosophy. Unix distinguishes itself from its predecessors as the first portable operating system: Almost the entire operating system is written in the C programming language, thus allowing Unix to reach numerous platforms. †

Unix philosophy : A set of cultural norms and philosophical approaches to minimalist, modular software development. It is based on the experience of leading developers of the Unix operating system. Early Unix developers were important in bringing the concepts of modularity and reusability into software engineering practice, spawning a “software tools” movement. Over time, the leading developers of Unix established a set of cultural norms for developing software; these norms became as important and influential as the technology of Unix itself, leading to the “Unix philosophy.” The Unix philosophy emphasizes building simple, short, clear, modular, and extensible code that can be easily maintained and repurposed by developers other than its creators. The Unix philosophy favors composability as opposed to monolithic design. † ℹ︎ is.gd/oOiBbo

UPA : → Usability Professionals Association

Upstream : In version control, the parent repository, that is, the repository that was used to clone or fork the current work repository.

URC : → Uniform Resource Characteristic

Urchin Tracking Module : A set of URL parameters used by marketers to track the effectiveness of online marketing campaigns across traffic sources and publishing media. The UTM parameters were introduced by Google Analytics’ predecessor Urchin and, consequently, are supported out-of-the-box by Google Analytics. The UTM parameters include utm_source, utm_medium, utm_campaign, utm_term, and utm_content. †

URI : → Uniform Resource Identifier

URL : → Uniform Resource Locator

URL encoding : A mechanism for encoding information in a Uniform Resource Identifier (URI) under certain circumstances. Although it is known as URL encoding, it is, in fact, used more generally within the main Uniform Resource Identifier (URI) set, which includes both Uniform Resource Locator (URL) and Uniform Resource Name (URN). As such, it is also used in the preparation of data of the application/x-www-form-urlencoded media type, as is often used in the submission of HTML form data in HTTP requests. With URL encoding, when a reserved character has a special meaning in a certain context, and a URI scheme says that it is necessary to use that character for some other purpose, then the character must be percent-encoded. For example, / becomes %2F. †

URL forwarding : → Redirect

URL redirection : → Redirect

URL slug : → Slug

URN : → Uniform Resource Name

Usability : The ease of use and learnability of a human-made object such as a tool or device. In software engineering, usability is the degree to which a software can be used by specified consumers to achieve quantified objectives with effectiveness, efficiency, and satisfaction in a quantified context of use. Usability includes methods of measuring usability, such as needs analysis and the study of the principles behind an object’s perceived efficiency or elegance. In human-computer interaction and computer science, usability studies the elegance and clarity with which the interaction with a computer program or a website (web usability) is designed. Usability considers user satisfaction and utility as quality components, and aims to improve user experience through iterative design. †

Usability inspection : The name for a set of methods where an evaluator inspects a user interface. This is in contrast to usability testing where the usability of the interface is evaluated by testing it with real users. Usability inspections can generally be used early in the development process by evaluating prototypes or specifications for the system that cannot be tested with users. Usability inspection methods are generally considered to be cheaper to implement than testing with users. They include cognitive walkthroughs (task-specific), heuristic evaluations (holistic), and pluralistic walkthroughs. †

Usability Professionals Association : → User Experience Professionals Association

Usability testing : A technique used in user-centered interaction design to evaluate a product by testing it with users. This can be seen as an irreplaceable usability practice, since it gives direct input on how real users use the system. Usability testing is concerned with the design intuitiveness of the product, tested with users who have no prior exposure to it. Such testing is paramount to the success of an end product as a fully functioning product that creates confusion among its users may not last as long as one that is easy to use. Usability testing stands in contrast with usability inspection methods that do not directly involve users. †

Usenet : A worldwide distributed discussion system. Usenet was developed from the general-purpose Unix-to-Unix Copy (UUCP) dial-up network architecture. Tom Truscott and Jim Ellis conceived the idea in 1979, and it was established in 1980. Users read and post messages (called articles or posts, and collectively termed news) to one or more categories, known as newsgroups. Usenet resembles a bulletin board system (BBS) in many respects and is the precursor to Internet forums that are widely used today. Discussions are threaded, as with web forums and BBSs, though posts are stored on the server sequentially. The name comes from the term “users network.” †

User : A person who utilizes a computer or network service. Users of computer systems and software products often lack the technical expertise required to fully understand how they work. Power users use advanced features of programs, though they are not necessarily capable of computer programming and system administration. Some software products provide services to other systems and have no direct end users. End users are the ultimate human users (also referred to as operators) of a software product. The term is used to abstract and distinguish those who only use the software from the developers of the system, who enhance the software for end users. †

User acceptance testing : → Acceptance testing

User agent : Software (a software agent) that is acting on behalf of a user, such as a web browser that retrieves, renders, and facilitates user interaction with web content. An email reader is a mail user agent. In many cases, a user agent acts as a client in a network protocol used in communications within a client-server distributed computing system. †

User Agent Accessibility Guidelines : A set of guidelines for user agent developers (such as web browsers and media players) aimed at making user agents accessible to users with disabilities. UAAG 1.0 became a W3C Recommendation in 2002. † ℹ︎ w3.org/TR/UAAG20

User agent sniffing : → Browser sniffing

User agent string : A characteristic string that lets servers and network peers identify the application, operating system, vendor, or version of the requesting user agent. The common format for web browsers is User-Agent: Mozilla/5.0 (<system-information>) <platform> (<platform-details>) <extensions>. ‡

User Datagram Protocol : One of the core members of the Internet protocol suite. The protocol was designed in 1980 by David P. Reed and formally defined in RFC 768. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol (IP) network. Prior communications are not required in order to set up communication channels or data paths. UDP is suitable for purposes where error checking and correction are either not necessary or are performed in the application; UDP avoids the overhead of such processing in the protocol stack. Time-sensitive applications often use UDP because dropping packets is preferable to waiting for packets delayed due to retransmission, which may not be an option in a real-time system. †

User Experience : A person’s emotions and attitudes about using a particular product, service, or system. UX includes the practical, experiential, affective, meaningful, and valuable aspects of human-computer interaction and product ownership. Additionally, it includes a person’s perceptions of system aspects such as utility, ease of use, and efficiency. User experience may be subjective in nature to the degree that it is about individual perception and thought with respect to a product or system. User experience varies dynamically, constantly modifying over time due to changing usage circumstances. Simplified, user experience is about how a user interacts with and experiences a product. †

User Experience Professionals Association : An organization of domain experts “who research, design, and evaluate the user experience (UX) of products and services.” The UXPA was founded as the Usability Professionals Association in 1991, and changed their name to “User Experience Professionals Association” in 2012. ℹ︎ uxpa.org

User Interface : The space of interaction between humans and machines. The goal of this interaction is to allow effective operation and control of the machine from the human end, whilst the machine simultaneously feeds back information that aids the operators’ decision-making process. Examples of this broad concept of user interfaces include the interactive aspects of computer operating systems, hand tools, heavy machinery operator controls, and process controls. The design considerations applicable when creating user interfaces are related to or involve such disciplines as ergonomics and psychology. Generally, the goal of user interface design is to produce a user interface which makes it easy, efficient, and enjoyable (user-friendly) to operate a machine in the way which produces the desired result. †

User interface markup language : A markup language that renders and describes graphical user interfaces and controls. Many of these markup languages are dialects of XML and are dependent upon a pre-existing scripting language engine, usually a JavaScript engine, for rendering of controls and extra scriptability. The concept of user interface markup languages is primarily based upon the desire to prevent the “re-invention of the wheel” in the design, development, and function of a user interface. The typical user interface markup language solidifies often re-used program or script code in the form of markup, making it easier to focus upon design of a user interface in an understandable dialect as opposed to focus on function. Examples for UIMLs include XAML, XUL, and SVG. †

User JavaScript : → Userscript

User story : An informal, natural language description of one or more features of a software system. User stories are often written from the perspective of an end user or user of a system. They are often recorded on index cards, on Post-it notes, or in project management software. Depending on the project, user stories may be written by various stakeholders including clients, users, managers, or development team members. †

User testing : → Usability testing

User-Centered Design : A framework of processes in which usability goals, user characteristics, environment, tasks and workflow of a product, service, or process are given extensive attention at each stage of the design process. User-centered design can be characterized as a multi-stage problem-solving process that not only requires designers to analyze and envision the way users are likely to consume a product, but also to validate their assumptions with regard to the user behavior in real world tests. These tests are conducted with or without actual users during each stage of the process ranging from requirements to pre-production to post-production, ensuring that development proceeds with the user as the center of focus. Such testing is necessary as it is often difficult for the designers of a product to understand intuitively what a first-time user of their design experiences, and what each user’s learning curve may look like. User-centered design is considered to lead to increased product usefulness and usability. †

User-Driven Development : → User-Centered Design

Userscript : A program, usually written in JavaScript, for modifying web pages to augment browsing. Uses include setting up keyboard shortcuts, adding features to sites, or enhancing the browsing history. Userscripts are often referred to as Greasemonkey scripts, named after the original userscript manager for the Firefox browser. †

Using Declarations Just Once : A CSS methodology to use each declaration only one time (per section, component, or file, within media query bounds). UDJO helps limit repetition (Don’t Repeat Yourself) and reduces the need and use for custom properties (CSS variables). UDJO was first mentioned in 2008. ℹ︎ meiert.com/en/blog/dry-css

USP : → Unique Selling Point : → Unique Selling Proposition

UTF-8 : A variable-width character encoding capable of encoding all 1,112,064 valid code points in Unicode using one to four 8-bit bytes. The encoding is defined by the Unicode Standard, and was originally designed by Ken Thompson and Rob Pike. The name is derived from Unicode (or Universal Coded Character Set) Transformation Format 8-Bit. UTF-8 was designed for backward-compatibility with ASCII. Since 2009, UTF-8 has been the dominant encoding (of any kind, not just of Unicode encodings) for the World Wide Web (and declared mandatory “for all things” by WHATWG) and as of January 2020 accounts for 94.8% of all web pages (some of which are simply ASCII, as it is a subset of UTF-8) and 96% of the top 1,000 highest ranked web pages. †

UTF-16 : A character encoding capable of encoding all valid code points of Unicode. The encoding is variable-length, as code points are encoded with one or two 16-bit code units. UTF-16 arose from an earlier fixed-width 16-bit encoding known as UCS-2 (for 2-byte Universal Character Set) once it became clear that more than 216 code points were needed. UTF-16 is used internally by systems such as Microsoft Windows, Java, and JavaScript. It is also often used for plain-text as well as for word-processing data files on Windows. It is rarely used for files on Unix/Linux or macOS. It never gained popularity on the Web, where UTF-8 is dominant. †

UTF-32 : A fixed-length encoding used to encode Unicode code points that uses exactly 32 bits (4 bytes) per code point. UTF-32 is a fixed-length encoding, in contrast to all other Unicode transformation formats, which are variable-length encodings. Each 32-bit value in UTF-32 represents one Unicode code point and is exactly equal to that code point’s numerical value. The main advantage of UTF-32 is that the Unicode code points are directly indexed. Finding the nth code point in a sequence of code points is a constant time operation. In contrast, a variable-length code requires sequential access to find the nth code point in a sequence. The main disadvantage of UTF-32 is that it is space-inefficient, using 4 bytes per code point, including 11 bits that are always zero. †

Utility class : In software development, a special case of a helper class in which the methods are all static. † : → Helper class

UTM : → Urchin Tracking Module

UUID : → Universally unique identifier

UVP : → Unique Value Proposition

UX : → User Experience

UXPA : → User Experience Professionals Association

UXSS : → Universal XSS

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