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Appendix A: Glossary (Normative)

This section is normative.


shortened form of a word, phrase, or name

Note: Includes initialisms and acronyms.


abbreviation made from the initial letters of a name or phrase that contains several words

Note: Many acronyms can be pronounced as words.

Example: NOAA is an acronym made from the initial letters of the National Oceanic and Atmospheric Administration in the United States.

activity where timing is essential

activity where timing is part of the design of the activity and removal of the time dependency would change the functionality of the content

alternate version

version that provides all of the same information and functionality and is as up to date as any non-conformant content

analog, time-dependent input

input whose result is different depending on the rate of the analog movement (such as when line width varies with pen speed or pressure.)

Note: Most actions carried out by a pointing device can also be done from the keyboard (for example, clicking, selecting, moving, sizing). However, there is a small class of input that is done with a pointing device that cannot be done from the keyboard in any known fashion. This type of input can be best characterized by the fact that the outcome can only be achieved by moving the pointer in a smooth fashion at a certain rate. For example, in a watercolor program stroke width and opacity may depend on the rate of movement (and/or pressure) of a "brush".

Application Programming Interface (API)

definitions of how communication may take place between applications

Note 1: Implementing APIs that are independent of a particular operating environment (as are the W3C DOM Level 2 specifications) may reduce implementation costs for multi-platform user agents and promote the development of multi-platform assistive technologies. Implementing conventional APIs for a particular operating environment may reduce implementation costs for assistive technology developers who wish to interoperate with more than one piece of software running on that operating environment.

Note 2: A "device API" defines how communication may take place with an input or output device such as a keyboard, mouse, or video card.

Note 3: In this document, an "input/output API" defines how applications or devices communicate with a user agent. As used in this document, input and output APIs include, but are not limited to, device APIs. Input and output APIs also include more abstract communication interfaces than those specified by device APIs. A "conventional input/output API" is one that is expected to be implemented by software running on a particular operating environment. For example, the conventional input APIs of the user agent are for the mouse and keyboard. For touch screen devices or mobile devices, conventional input APIs may include stylus, buttons, and voice. The graphical display and sound card are considered conventional output devices for a graphical desktop computer environment, and each has an associated API.

Note 4: This definition is based on User Agent Accessibility Guidelines 1.0 Glossary.


picture created by a spatial arrangement of characters (typically from the 95 printable characters defined by ASCII).

Assistive technology (in the context of this document)

a user agent that:

  1. relies on services (such as retrieving Web content and parsing markup) provided by one or more other "host" user agents. Assistive technologies communicate data and messages with host user agents by using and monitoring APIs.

  2. provides services beyond those offered by the host user agents to meet the requirements of users with disabilities. Additional services include alternative renderings (e.g., as synthesized speech or magnified content), alternative input methods (e.g., voice), additional navigation or orientation mechanisms, and content transformations (e.g., to make tables more accessible).

Example: Examples of assistive technologies that are important in the context of this document include the following:

  • screen magnifiers, which are used by people with visual disabilities to enlarge and change colors on the screen to improve the visual readability of rendered text and images;

  • screen readers, which are used by people who are blind or have reading disabilities to read textual information through synthesized speech or braille displays;

  • voice recognition software, which may be used by people who have some physical disabilities;

  • alternative keyboards, which are used by people with certain physical disabilities to simulate the keyboard;

  • alternative pointing devices, which are used by people with certain physical disabilities to simulate mouse pointing and button activations.

Note: This definition is based on User Agent Accessibility Guidelines 1.0 Glossary.

audio description

narration added to the soundtrack to describe important visual details that cannot be understood from the main soundtrack alone

Note 1: Audio descriptions of video provide information about actions, characters, scene changes, and on-screen text.

Note 2: In standard audio description, narration is added during existing pauses in dialogue. (See also Extended audio descriptions.)

authored component

an authored unit intended to be used as a part of another authored unit

authored unit

set of material created as a single body by an author

Example 1: a collection consisting of markup, a style sheet, and an image or audio clip.

Example 2: a set of Web pages intended to be viewed only as a unit or in sequence.

Note: This definition is based on Glossary of Terms for Device Independence.


set of technologies assumed to be supported by, and enabled in, user agents

Note: For more information on baselines and their use, refer to Technology Assumptions and the "baseline."


turn on and off between 0.5 and 3 times per second


text presented and synchronized with multimedia to provide not only the speech, but also sound effects and sometimes speaker identification

Note: In some countries, the term "subtitle" is used to refer to dialogue only and "captions" is used as the term for dialogue plus sounds and speaker identification. In other countries, subtitle (or its translation) is used to refer to both.

changes of context

change of :

  1. user agent;

  2. viewport;

  3. focus;

  4. content that changes the meaning of the Web unit.

Note: A change of content is not always a change of context. Small changes in content, such as an expanding outline or dynamic menu, do not change the context.


information to be communicated to the user by means of a user agent

Note: This includes the code and markup that define the structure, presentation, and interaction, as well as text, images, and sounds that convey information to the end-user.

context-sensitive help

help text that provides information related to the function currently being performed


a sudden, unexpected situation or occurrence that requires immediate action to preserve health, safety, or property

event handler

section of code that responds to an action taken by the user (or user agent)

Note: On Web pages, events are usually user actions such as moving the mouse, typing, etc.

  • An event handler determines the response to that action.

  • A device-specific event handler only responds to an action by one kind of input device.

  • An abstract event handler is one which can be activated by a variety of input devices.

extended audio descriptions

audio descriptions that are added to an audiovisual presentation by pausing the video so that there is time to add additional description

Note: This technique is only used when the sense of the video would be lost without the additional audio description.

full multimedia text alternative including any interaction

document including correctly sequenced descriptions of all visual settings, actions, and non-speech sounds combined with descriptive transcripts of all dialogue and a means of achieving any outcomes that are achieved using interaction during the multimedia

Note: A screenplay used to create the multimedia content would meet this definition only if it was corrected to accurately represent the final multimedia after editing.


processes and outcomes achievable through user action

general flash threshold
  • A sequence of flashes or rapidly changing image sequences where all three of the following occur:

    1. the combined area of flashes occurring concurrently (but not necessarily contiguously) occupies more than one quarter of any 341 x 256 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 x 768 pixels;

    2. there are more than three flashes within any one-second period; and

    3. the flashing is below 50 Hz.

Note 1: For the general flash threshold, a flash is defined as a pair of opposing changes in brightness of 10% or more of full scale white brightness, where brightness is calculated as 0.2126 * ((R / FS) ^ 2.2) + 0.7152 * ((G / FS) ^ 2.2) + 0.0722 * ((B / FS) ^ 2.2). R, G, and B are the red, green, and blue RGB values of the color; FS is the maximum possible full scale RGB value for R, G, and B (255 for eight bit color channels); and the "^" character is the exponentiation operator. An "opposing change" is an increase followed by a decrease, or a decrease followed by an increase. This applies only when the brightness of the darker image is below .80 of full scale white brightness.

Note 2: Based on Wisconsin Computer Equivalence Algorithm for Flash Pattern Analysis (FPA)


phrase whose meaning cannot be deduced from the meaning of the individual words and the specific words cannot be changed without losing the meaning

Example 1: In English, "kicking the bucket" means "dying". But the phrase cannot be changed to "kicking the buckets" or "kicking the tub" or "booting the bucket" or "knocking over the bucket" without losing its meaning.

Example 2: In English, "spilling the beans" means "revealing a secret." However, "knocking over the beans" or "spilling the vegetables" does not mean the same thing."

Example 3: In Japanese, the phrase "さじを投げる(どうするこ ともできなくなり、あきらめること" literally translates into "he threw a spoon". But it means that there was nothing he could do and finally he gave up.

Example 4: In Dutch, "Hij ging met de kippen op stok" literally translates into "He went to roost with the chickens". But it means that he went to bed early.

  1. a message to be sent and received

  2. a collection of facts or data from which inferences may be drawn

information that is conveyed by color

information presented in a manner that depends entirely on the ability to perceive color


for information purposes and not required for conformance

Note: Content required for conformance is referred to as "normative."


shortened form of a name or phrase made from the initial letters of words or syllables contained in that name or phrase

Note: Not defined in all languages.

Example 1: SNCF is a French initialism that contains the initial letters of the Sociétè Nationale des Chemins de Fer, the French national railroad.

Example 2: ESP is an initialism for extrasensory perception.

input error

information provided by the user that is not accepted

Note: This includes:

  1. Information that is required by the Web unit but omitted by the user

  2. Information that is provided by the user but that falls outside the required data format or values.


words used in a particular way by people in a particular field

Example: The word StickyKeys is jargon from the field of assistive technology/accessibility.

keyboard interface

interface used by software to obtain keystroke input

Note 1: Allows users to provide keystroke input to programs even if the native technology does not contain a keyboard.

Example: A touch screen PDA has a keyboard interface built into its operating system as well as a connector for external keyboards. Applications on the PDA can use the interface to obtain keyboard input either from an external keyboard or from other applications that provide simulated keyboard output, such as handwriting interpreters or speech to text applications with "keyboard emulation" functionality.

Note 2: Operation of the application (or parts of the application) through a keyboard operated mouse emulator, such as MouseKeys, does not qualify as operation through a keyboard interface because operation of the program is through its pointing device interface - not through its keyboard interface.


text, image, or sound that is presented to a user to identify a component within Web content

live audio-only

A time-based live presentation that contains only audio (no video and no interaction).

live video-only

A time-based live presentation that contains only video (no audio and no interaction).

Lower secondary education level

the two or three year period of education that begins after completion of six years of school and ends nine years after the beginning of primary education.

Note: This definition is based on [UNESCO].

luminosity contrast ratio

(L1 + 0.05) / (L2 + 0.05), where L1 is the luminosity of the lighter of the text or background colors, and L2 is the luminosity of the darker of the text or background colors.

Note 1: The luminosity of a color is defined as 0.2126 * ((R / FS) ^ 2.2) + 0.7152 * ((G / FS) ^ 2.2) + 0.0722 * ((B / FS) ^ 2.2).

  • R, G, and B are the red, green, and blue RGB values of the color.

  • FS is the maximum possible full scale RGB value for R, G, and B (255 for eight bit color channels).

  • The "^" character is the exponentiation operator.

Note 2: Luminosity values can range from 0 (black) to 1 (white), and luminosity contrast ratios can range from 1 to 21.


process or technique for achieving a result


audio or video synchronized with another type of media and/or with time-based interactive components


text by which software can identify a component within Web content to the user

Note: The name may be hidden and only exposed by assistive technology, whereas a label is presented even without assistive technology. In many (but not all) cases, the label is a display of the name.

natural languages

languages used by humans to communicate, including spoken, written, and signed languages

Note: See also sign language interpretation.

non-text content

content that is not represented by a Unicode character or sequence of Unicode characters when rendered in a user agent according to the formal specification of the content type

Note: This includes ASCII Art, which is a pattern of characters.


required for conformance

Note 1: One may conform in a variety of well-defined ways to this document.

Note 2: Content identified as "informative" or "non-normative" is never required for conformance.

parsed unambiguously

parsed into only one data structure

Note: Parsing transforms markup or other code into a data structure, usually a tree, which is suitable for later processing and which captures the implied hierarchy of the input.


stopped by user request and not restarted until requested by user


rendering of the content and structure in a form that can be perceived by the user

primary education level

six year time period that begins between the ages of five and seven, possibly without any previous education

Note: This definition is based on [UNESCO].

programmatically determined

determined by software from data provided in a user-agent-supported manner such that the user agents can extract and present this information to users in different modalities

programmatically set

set by software using methods that are user-agent-supported

pure decoration

serving only an aesthetic purpose, providing no information, and having no functionality.

real-time event

event that a) occurs at the same time as the viewing, b) is not completely generated by the content, and c) is not pre-recorded

Example 1: A Webcast of a live performance.

Example 2: An on-line auction with people bidding.

Example 3: Live humans interacting in a fantasy world using avatars.

red flash threshold
  • transition to or from a saturated red where all three of the following occur:

    1. The combined area of flashes occurring concurrently occupies more than one quarter of any 341 x 256 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 x 768 pixels.

    2. There are more than three flashes within any one-second period.

    3. The flashing is below 50 Hz.

regular expression

regular expression as defined in XML Schema Part 2: Datatypes, Appendix F.


text or a number by which software can identify the function of a component within Web content

Example: A number that indicates whether an image functions as a hyperlink, command button, or check box.

same functionality

identical result when used

Example: A submit "search" button on one Web page and a "find" button on another Web page may both have a field to enter a term and list topics in the web site related to the term submitted. In this case, they would have the same functionality but would not be labeled consistently.

same relative order

same position relative to other items

Note: Items are considered to be in the same relative order even if other items are inserted or removed from the original order. For example, expanding navigation menus may insert an additional level of detail or a secondary navigation section may be inserted into the reading order.

sign language interpretation

translation of spoken words and other audible information into a language that uses a simultaneous combination of handshapes, facial expressions, and orientation and movement of the hands, arms, or body to convey meaning

Note: Although some languages have a signed counterpart, most sign languages are independent languages that are unrelated to the spoken language of the same country or culture.

specific sensory experience

a sensory experience that is not purely decorative and does not primarily convey important information or perform a function

  1. The way the parts of an authored unit are organized in relation to each other; and

  2. The way a collection of Web units is organized

supplemental content

additional content, which users may use in addition to or instead of the default content, that illustrates or clarifies the default content

Example: Examples of supplemental content may include text, images and audio.


markup language, programming language, style sheet, data format, or API

test or exercise that must use a particular sense

test where the content must be presented in a particular sensory format

Example: Color blindness test, hearing test, vision exercise, spelling test.


sequence of characters

Note: Characters are those included in the Unicode/ISO/IEC 106464 repertoire.

text alternative

programmatically determined text that is used in place of non-text content, or text that is used in addition to non-text content and referred to from the programmatically determined text


universal character set that defines all the characters needed for writing the majority of living languages in use on computers

Note 1: For more information, refer to the Unicode Consortium or to the tutorial entitled, "Character sets & encodings in XHTML, HTML and CSS" produced by the W3C Internationalization Activity.

Note 2: This definition is based on [UNICODE]

used in an unusual restricted way

words used in such a way that users must know exactly what definition to apply in order to understand the content correctly

Example: The word "representational" means something quite different if it occurs in a discussion of visual art as opposed to a treatise on government, but the appropriate definition can be determined from context. By contrast, the word "text" is used in a very specific way in WCAG 2.0, so a definition is supplied in the glossary.

user agent

any software that retrieves and renders Web content for users

Example: Web browsers, media players, plug-ins, and other programs — including assistive technologies — that help in retrieving and rendering Web content.


implemented by user agents and assistive technologies

Note: One of the factors that should be considered before adding a technology to a baseline is the availability of affordable user agents and assistive technologies which support the technology.

variations in presentations of text

changes in the visual appearance or sound of the text, such as changing to a different font or a different voice


the technology of moving pictures or images

Note: Video can be made up of animated or photographic images, or both.

Web unit

a collection of information, consisting of one or more resources, intended to be rendered together, and identified by a single Uniform Resource Identifier (such as URLs)

Note: This definition is based on the definition of Web page in Web Characterization Terminology & Definitions Sheet. The concept of simultaneity was removed to allow the term to cover interactive and scripted content.

Example 1: An interactive movie-like shopping environment where the user navigates about and activates products to have them demonstrated, and moves them to a cart to buy them.

Example 2: A Web page including all embedded images and media.

Wisconsin Computer Equivalence Algorithm for Flash Pattern Analysis (FPA)

a method developed at the University of Wisconsin, working in conjunction with Dr. Graham Harding and Cambridge Research Associates, for applying the United Kingdom's "Ofcom Guidance Note on Flashing Images and Regular Patterns in Television (Re-issued as Ofcom Notes 25 July 2005)" to content displayed on a computer screen, such as Web pages and other computer content

Note: The Ofcom Guidance Document [OFCOM] is based on the assumption that the television screen occupies the central ten degrees of vision. This is not accurate for a screen which is located in front of a person. The Wisconsin algorithm basically carries out the same analysis as the Ofcom Guidelines except that is does it on every possible ten degree window for a prototypical computer display.