Glossary

Above the fold

Pertaining to content that is visible without scrolling when the visitor first lands on a web page.

accessibility

Refers to building web sites, applications, and pages so that there are as few barriers to use as possible for anyone, regardless of ability and the device used to access the information.

AIFF

Audio Interchange File Format. Standard audio format originally developed for the Macintosh, which is now supported on PCs as well. It is one of the formats commonly used for distributing audio on the Web.

Ajax

A web development technique for creating interactive web applications using a combination of (X)HTML, CSS, the DOM, and the nonstandard XMLHttpRequest object to exchange data asynchronously with the server. Because the page doesn't need to refresh with each user interaction, Ajax makes web applications feel more like desktop applications.

alpha channel

In graphics formats, an extra channel for storing information about an image. The alpha channel works like a mask that applies properties (such as transparency) to the pixels in the image. Other channels typically include color value informationas in the red, green, and blue channels of an RGB image.

alpha-channel transparency

The method of transparency used by 24-bit PNGs, which use an additional (alpha) channel to store variable levels of transparency (up to 256) for each pixel in the image.

animated GIF

A GIF89a that contains multiple frames and a "control block" for controlling the animation timing and display.

Apache

A popular open source (free) web server.

applet

A self-contained mini-executable program, such as one written in the Java programming language.

ASCII (American Standard Code for Information Interchange)

A coded character set that includes 128 characters mostly from the Roman alphabet used in modern English.

ASP

Active Server Pages. The part of Microsoft's Internet Information Server software that allows server-side scripting for the creation of dynamically generated web pages and database functions. Web pages created with ASP commonly have the suffix .asp.

attribute minimization

The SGML practice in which certain attributes can be reduced to just the attribute value. XML does not support minimization, so all the attributes have to be explicitly declared (for example, checked="checked").

audio bit depth

The number of bits used to define the resolution of the amplitude (or volume) of a digital audio waveformthe more bits, the more accurate the rendering of the original audio source and the larger the resulting audio file. Some common bit depths are 8-bit (which sounds thin or tinny, like a telephone signal) and 16-bit, which is required to describe music of CD quality.

AVI

Audio/Video Interleaved. A digital video format developed by Microsoft in which audio and video information are interleaved in every frame for smoother playback.

Basic Multilingual Plane (BMP)

The first 16 bits, or 65,536 positions in Unicode, which includes most of the common characters used in the languages of the world, such as character sets for Latin, Greek, Cyrillic, hirgana, katakana, and others, as well as mathematical and other miscellaneous characters.

behavioral layer

A term used to refer to the interactivity added to a web page via scripting (usually in JavaScript). See also structural layer and presentational layer.

binary files

Files made up of compiled data (ones and zeros), such as executable programs, graphic images, movies, etc. Some programs refer to the binary mode as "raw data" or "image data."

block-level elements

Elements that start a new line and tend to stack up like blocks in the normal flow of the document. Block elements make up the main components of document structure.

cascade

In CSS, this refers to a hierarchical system for handling conflicting style sheets that assigns different weights to various sources of style information.

CGI

Common Gateway Interface. The mechanism for communication between the web server and other programs (CGI scripts) running on the server.

character encoding

The method used to transform the character stream in a document to a byte stream that is interpreted by user agents.

character entity

An abbreviated name for a character that is predefined in a DTD for use in a markup language. Character entities are provided as a convenience for authors because they may be easier to remember than the Numeric Character Reference for the character.

character set

Any collection of characters that are used together for a particular function. Many character sets are standardized, such as Latin-1 (ISO-8859-1) and Unicode.

client

A software application that extracts services from a server somewhere on the network. A web browser is a client that renders and displays documents on remote servers.

CMYK

A four-channel color model describing Cyan, Magenta, Yellow, and Black ink colors. Images in CMYK mode are not appropriate for web graphics, which must be RGB.

code point

The numeric position of a character in a coded character set such as Unicode.

codec

Compression/decompression algorithms applied to media files.

combinator

In CSS, a specific character used to signify the type of relationship between the elements in a rule selector.

CSS (Cascading Style Sheets)

A style sheet language used to describe the presentation of documents written in HTML, XHTML, and other XML languages.

data rate

In video, the rate at which data must be transferred for the video to play smoothly without interruption. The data rate (also called "bit rate") for a movie is measured in kilobytes per second (K/s or KB/s). It can be calculated by dividing the size of the file (in K) by the length of the movie (in seconds).

declaration

In CSS, the portion of a style rule that contains the property and value to be applied to an element or set of elements.

deprecated

In the HTML 4.0 and 4.01 Recommendations, a label identifying an HTML element or attribute as "outdated" and discouraged from use in favor of newer constructs (most often CSS properties).

DHTML

Short for "Dynamic HTML," a bit of marketing jargon used to describe the integration of JavaScript, Cascading Style Sheets, and the Document Object Model. The term "DHTML" is falling out of favor because of its associations with an era of browser-sniffing and obtrusive scripting. The preferred (and more standards-oriented) term is now "DOM Scripting."

dithering

The approximation of a color by mixing pixels of similar colors that are available in the image palette. The result of dithering is a random dot pattern or noise in the image.

DOCTYPE declaration

Specifies the DTD used in an HTML or XHTML document. The DOCTYPE declaration is required for validation.

document character set

In SGML documents, this is the base character set for interpreting character references.

Document Object Model (DOM)

A platform- and language-neutral interface that allows programs and scripts to dynamically access and update the content, structure, and style of HTML and XML documents.

document tree

The hierarchical structure of a document established by the markup of elements and their relationship to one another.

dpi

Dots per inch. In graphics, this is the measurement of the resolution of a printed image. It is commonly (although incorrectly) used to refer to the screen resolution of web graphics, which is technically measured in ppi (pixels per inch). See also ppi.

Document Type Definition (DTD)

A document that defines the elements, attributes, and entities as well as the rules of their use in a markup language. A document that conforms to its DTD is said to be valid. The syntax for DTD definitions follows the rules of SGML.

DTD

See Document Type Definition.

Ecma (European Association for Standardizing Information and Communication Systems)

The Ecma manages information-technology standards, including ECMAScript, the standardized version of JavaScript.

encoding (audio)

The process of converting an analog source (such as an analog audio signal) into digital format. An encoder is the software that does the converting.

escaped character (or escaping)

A character that is represented by its character reference. The character reference may be numerical or a predefined character entity. In XML, XHTML, and HTML documents, escaped characters are preceded by & and end with ; (for example, &). In CSS, escaped characters are indicated by a backslash (\) and are terminated with a space (for example, \C7).

frame rate

In video, frames per second; used as a measure of video quality.

FTP

File Transfer Protocol. A protocol for moving files over the Internet from one computer to another. FTP is a client/server system: one machine must be running an FTP server; the other, an FTP client.

gamma

Refers to the overall brightness of a computer monitor's display. In technical terms, it is a numerical adjustment for the nonlinear relationship of voltage to light intensity.

GIF

Graphic Interchange Format. Common file format of web graphic images. GIF is a palette-based, 8-bit format that compresses images with the lossless LZW compression scheme. GIF is most appropriate for images with areas of flat color and sharp contrast. See also LZW compression.

hexadecimal

A base-16 numbering system consisting of the characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F, where A through F represent the decimal values 10 through 15. It is used in (X)HTML and XML to provide RGB color values and Numerical Character References that use hexadecimal Unicode code points.

HTML

Hypertext Markup Language. The markup language used for web documents.

HTTP

Hypertext Transfer Protocol. The protocol that defines how web pages and media are requested and transferred between servers and browsers.

HTTP header

Information about a document that the web server sends to the user agent along with the document when it is requested by a user agent.

i18n

The W3C abbreviation for "internationalization" ("i," 18 letters, then "n"), relating to efforts to ensure that the formats and protocols defined by the W3C are usable worldwide in all languages and writing systems.

IETF (The Internet Engineering Task Force)

An international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet as a whole. It publishes Request for Comments (RFCs) that define how things are done over the Internet, including FTP, TCP/IP, HTTP, and email.

image map

A single image that contains multiple hypertext links.

indexed color

In graphics, a system for rendering colors in 8-bit images. Indexed color files, such as GIFs, contain an index (also called a palette or color lookup table) of colors and associated index numbers, which is used to render color in the image.

inheritance

In CSS, the concept by which styles are passed down from an element to its descendants. A child element is said to inherit property values from its parent.

inline elements

Elements that occur in the flow of text and do not cause line breaks by default.

ISO (International Organization for Standardization)

The standards organization that manages more than 10,000 international standards for everything from information systems to manufacturing specifications. Their seal of approval helps keep commerce and information technologies compatible worldwide.

Java

A cross-platform, object-oriented programming language developed by Sun Microsystems. It can be used to create whole applications; however, its primary contribution to the Web has been in the form of Java applets: self-contained, mini-executable programs.

JavaScript

A client-side scripting language originally developed by Netscape and later standardized as ECMAScript that adds interactivity and conditional behavior to web pages. It has little in common with Java.

JPEG

A lossy compression algorithm developed by the Joint Photographic Experts Group. It is used by files in the JFIF format, which are commonly referred to as "JPEG files." JPEG is most efficient at compressing images with gradations in tone and no sharp edge contrasts. Photographic images are typically best saved in JPEG format.

key frames

In video, master frames placed throughout a video against which the following frames are compared (for use with temporal, or interframe, compression).

Linux

A version of Unix designed to run on PCs.

lossy compression

A method for reducing file size in which some data (usually indiscernible to human perception) is deleted to achieve a higher compression rate.

lossless compression

A method for reducing the size of a file without loss of data; in lossless compression, redundant information is removed.

LZW compression

Short for Lempel-Zev-Welch, the names of the inventors. A lossless compression scheme that takes advantage of repetition in data streams (such as a row of pixels of identical color). It is the compression scheme used by graphic files in the GIF format.

MathML

Math Markup Language. An XML application for describing mathematical notation and capturing its structure and content.

MIDI

Musical Instrument Digital Interface. This audio format uses numerical commands to describe the pitch and endurance of notes that are "played" by available digital instrument sounds.

MIME types

Multimedia Internet Mail Extensions. A protocol that defines a number of content types and subtypes and allows programs like web browsers, news readers, and email clients to recognize different kinds of files and deal with them appropriately. The MIME type specifies what media a file is, such as an image, audio, or video, and the subtype identifies the precise file format.

MP3

Audio file format (MPEG I, Level-III) capable of high levels of compression with little discernible loss of quality. It has become the standard for sharing audio files over the Internet.

MPEG

A family of multimedia standards created by the Motion Picture Experts Group, commonly used to refer to audio and video files saved using one of the MPEG compression schemes.

namespace

A uniquely named group of element and attribute names. XML documents refer to namespaces in order to prevent confusion between competing element names.

nonreplaced element

In CSS, an element whose text content is included in the source document, such as a heading or paragraph.

normal flow

The default left-to-right, top-to-bottom rendering of content in (X)HTML documents in left-to-right reading languages. The only way to remove an element from the normal flow is to float or position it using style rules. The normal flow for right-to-left reading languages is from right-to-left, top-to-bottom.

palette

A table in an 8-bit indexed color file (such as GIF) that provides color information for the pixels in the image.

PDF

Portable Document Format. A file format developed by Adobe Systems used for capturing formatted page layouts for distribution. PDF documents are viewed with the required Adobe Acrobat Reader.

PHP

Hypertext Preprocessor. An open source, server-side tool for creating dynamically generated web pages (similar to Microsoft's ASP).

PNG

Portable Network Graphic. A versatile graphics file format that features support for both 8-bit (PNG8) indexed images and 24-bit images (PNG24). PNGs also feature variable transparency levels, automatic color correction controls, and a lossless yet highly efficient compression scheme.

ppi

Pixels per inch. The measurement of the resolution of a screen image.

presentation

The way a document is displayed or delivered to the user, whether it's on a computer monitor, a cell phone display, or read aloud by a screen reader

presentational layer

A term used to refer to style information applied to elements in a document. In web documents, presentation is controlled by Cascading Style Sheets. See also structural layer and behavioral layer.

QuickTime

A system extension that makes it possible to view audio and video information on a computer. It was originally developed for the Macintosh but is now available for Windows machines as well, and has been adopted as the video standard by the ISO in their development of MPEG-4. The term also refers to the file format.

RDF

Resource Description Framework. An XML application used to define the structure of metadata for documents, i.e., data that is useful for indexing, navigating, and searching a site.

replaced element

In the CSS, an element whose content is not provided in the source document, but rather acts as a placeholder for content brought in from an external source (such as an img) or rendered by the user agent (such as most form controls).

RGB color

A color system that describes colors based on combinations of red, green, and blue light.

rollover

The act of passing the mouse pointer over an element's space, or the events triggered by that action (such as a changing style, image, or pop-up message, sometimes called rollover events).

root element

The element that contains all other elements, called the root element because it has no ancestors. In HTML and XHTML documents, the root element is html.

sampling rate

In a digital audio file, the number of samples taken per second.

selector

In CSS, the portion of a style rule that targets an element or set of elements for the application of style properties

semantic

Of or related to meaning. In terms of web authoring, documents should be marked up semantically, that is, choosing (X)HTML elements that accurately describe the meaning of their contents.

server

Any networked computer running software that enables it to answer requests for documents and other data.

Server Side Includes (SSI)

Special placeholders in an HTML document that the server is to replace with actual data just before sending the final document to the browser. Extended SSI (XSSI) (part of Apache 1.2 and higher) provides more advanced command functions, including conditional behaviors.

SGML

Standard Generalized Markup Language. A meta-language that provides a comprehensive set of syntax rules for marking up the structure of documents and data. HTML is a subset of SGML.

sIFR

Scalable Inman Flash Replacement. A technique for replacing short text elements in a web document with small Flash movies in order to achieve rich typography on web pages without sacrificing accessibility, search engine friendliness, or markup semantics.

SMIL

Synchronized Multimedia Integration Language, an XML-based language for creating multimedia, time-based presentation. SMIL combines audio, video, text, animation, and graphics in a precise, synchronized fashion.

spatial compression

In video, spatial compression is applied to each individual frame of the video, using compression schemes commonly used on still images (also called "intraframe" compression).

spatial frequency

Refers to the concentration of detail in an image. For example, an image of a blue sky would be considered to have low frequency. A detailed image, such as a close-up of blades of grass, has high frequency.

structural layer

A term used to describe the marked up content of the source document that provides structure of and serves as a foundation for presentation instructions (added with Cascading Style Sheets) and behaviors (added by a scripting language such as JavaScript). See also presentational layer and behavioral layer.

SVG

Standardized Vector Graphics. An XML language for defining two-dimensional vector graphics.

Telnet

An internet protocol for logging into and using a remote system on the Internet. Telnet is a client/server system that requires a Telnet server running on one computer and a Telnet client on the other.

temporal compression

In video, temporal compression takes place over a series of frames, deleting information that is repeated between frames (also called "interframe" compression).

Universal Character Set (UCS)

This is the document character set used by HTML, XHTML, and XML documents. It is defined by both the Unicode and ISO/IEC 10646 standards.

Unix

A multiuser, multitasking operating system developed by Bell Laboratories. It also provides programs for editing text, sending email, preparing tables, performing calculations, and many other specialized functions that normally require separate applications programs.

valid markup

In an XML application, markup that properly uses the elements and attributes as specified in a Document Type Definition (DTD).

validating parser

A parser that checks a document for conformance with its declared DTD.

W3C

The World Wide Web Consortium. A consortium of many companies and organizations that "exists to develop common standards for the evolution of the World Wide Web."

WAI

Web Accessibility Initiative. The committee at the World Wide Web Consortium (W3C) that ensures that web technologies are accessible to users with disabilities.

WAV

Waveform Audio File Format. This format was developed for the PC but is now supported on Macintosh as well.

web palette

The set of 216 colors that will not dither or shift when viewed with browsers on 8-bit monitors.

well-formed

Describes a marked up document that abides by the strict syntax rules of XML.

WML

Wireless Markup Language. An XML-based language for creating applications for wireless devices. It is part of the Wireless Application Protocol (WAP). WML is growing obsolete

XHTML

A reworking of the HTML 4.01 specification to abide by the rules and syntax of XML.

XML

Extensible Markup Language. A new standard for marking up documents and data. XML is based on SGML, but with a reduced feature set that is more appropriate for distribution via the Web. XML allows authors to create customized markup languages.

XML declaration

The first line in XML documents that specifies the version of XML used. The character encoding may also be specified. XML declarations are optional but recommended. For example, <?xml version="1.0" encoding="UTF-8"?>.

XML Schema

A method for defining the elements, attributes, and entities in an XML markup language. It is equivalent to a Document Type Definition (DTD), but it is written according to the syntax of XML (DTDs are based on SGML).

XSL

Extensible Style Language. A system for controlling the presentation of complex XML documents and structured data. It is more robust than Cascading Style Sheets.

XSLT (Extensible Stylesheet Language for Transformations)

A subset of XSL (Extensible Stylesheet Language), an XSLT style sheet is necessary when an XML document is "transformed" before final display, such as translating it from one XML language to another, or replacing certain content with other content.