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X10 n. A popular communications protocol for powerline carrier (PLC) systems that uses existing electrical wiring in a home or building for home networking. X10 uses RF signals to communicate between transmitters and receivers. See also home automation, home network, powerline carrier system.

X.200 n. See X series.

X.25 n. A recommendation published by the ITU-T (formerly CCITT) international communications standards organization that defines the connection between a terminal and a packet-switching network. X.25 incorporates three definitions: the electrical connection between the terminal and the network, the transmission or link-access protocol, and the implementation of virtual circuits between network users. Taken together, these definitions specify a synchronous, full-duplex terminal-to-network connection. Packet format, error control, and other features are equivalent to portions of the HDLC (High-level Data Link Control) protocol defined by the International Organization for Standardization (ISO). See also CCITT X series, HDLC, packet switching, virtual circuit.

X3D n. Acronym for 3D XML. An XML-based 3-D graphics specification incorporating the behavior capabilities of the Virtual Reality Modeling Language (VRML). X3D is compatible with existing VRML content and tools and supports full integration with other XML-based technologies. The X3D specification was developed and administered by the Web 3D Consortium.

X.400 n. See X series.

X.445 n. See X series.

X.500 n. See X series.

X.509 n. See X series.

X.75 n. See X series.

x86 n. Any computer based on an 8086, 80286, 80386, 80486, or Pentium microprocessor.

x-axis n. The horizontal reference line on a grid, chart, or graph that has horizontal and vertical dimensions. See also Cartesian coordinates.

Xbase n. A generic name for a family of database languages based on dBASE, a copyrighted product of the Ashton-Tate Corporation. Xbase languages have since developed characteristics of their own and are now only partly compatible with the dBASE family. Xbase primarily refers to three different file types (.dbf, .dbt, and .ndx). Also called: xBase, xbase, XBase.

Xbox n. A video game console developed by Microsoft Corporation and released in 2001. Powered by an Intel 733-MHz processor, the Xbox delivers increased graphics capability over previously released game consoles and provides extensive storage capacity for gaming information. Peripherals plug into four game controller ports. An Ethernet port enables online gaming via a broadband connection. See also computer game, console game, GameCube, PlayStation. Compare Dreamcast.

X button n. See close button.

XCMD n. Short for external command. An external code resource used in HyperCard, a hypermedia program developed for the Macintosh. See also HyperCard, XFCN.

X Consortium n. The body, composed of several hardware firms, that governed the standards for the X Window System. The Open Group s X Project Team now has responsibility for the X Window System. See also X Window System.

xDSL n. An umbrella term for all of the digital subscriber line (DSL) technologies, which use a variety of modulation schemes to pack data onto copper wires. The x is a placeholder for the first or first two letters of a member technology, which might be ADSL, HDSL, IDSL, RADSL, or SDSL. See also DSL.

XENIX n. A version of UNIX that was originally adapted by Microsoft for Intel-based personal computers. Although it has been sold by many vendors, including Microsoft, Intel, and the Santa Cruz Operation (SCO), it has become principally identified with SCO. See also UNIX.

xerography n. See electrophotography.

Xerox Network System n. See XNS.

Xerox PARC n. Short for Xerox Palo Alto Research Center. Xerox s research and development facility in Palo Alto, California. Xerox PARC is the birthplace of such innovations as the local area network (LAN), the laser printer, and the graphical user interface (GUI).

XFCN n. Short for external function. An external code resource that returns a value after it has completed executing. XFCNs are used in HyperCard, a hypermedia program developed for the Macintosh. See also HyperCard, XCMD.

XFDL n. Short for Extensible Forms Description Language. a document description language introduced and submitted to the World Wide Web Committee in 1998 by the Canadian Internet forms company UWI.Com. XFDL is an XML-based language for describing complex forms, such as legal and government documents. It is designed to allow for interactivity yet remain consistent with Internet standards.

XGA n. See Extended Graphics Array.

x-height n. In typography, the height of the lowercase letter x in a particular font. The x-height thus represents the height of the body only of a lowercase letter, excluding ascenders (such as the top of the letter b) and descenders (such as the tail on the letter g). See also ascender, descender.

XHTML n. Short for Extensible Hypertext Markup Language. A markup language incorporating elements of HTML and XML. Web sites designed using XHTML can be more readily displayed on handheld computers and digital phones equipped with microbrowsers. XHTML was released for comments by the World Wide Web Consortium (W3C) in September 1999. See also HTML, microbrowser, XML.

XIP n. See execute in place.

XLANG n. A derivative XML language that describes the logical sequencing of business processes, as well as the implementation of the business process by using various application services.

XLink n. An XML language that provides a set of attributes that are used to create links between resources. XLink provides complex extended linking, link behavior, and management capabilities. XLink is able to describe links that connect sets of resources, point to multiple targets, or serve multiple roles within an XML document.

XLL n. Acronym for eXtensible Linking Language. Broad term intended to denote the family of XML linking/pointing/addressing languages, which include XLink, XPointer, and XPath.

XMI n. 1. Acronym for XML Metadata Interchange Format. An object-based model for exchanging program data across the Internet. XMI is sponsored by IBM, Unisys, and others and was submitted as a proposed standard to the Object Management Group (OMG); it is now one of OMG s recommended technologies. XMI is designed to allow for storing and sharing programming information and exchanging data among tools, applications, and storage locations through a network or the Internet so that software developers can collaborate on applications, even if they are not all using the same development tools. 2. As XMI bus, a 64-bit parallel bus supported on certain DEC and Alpha Server processors. An XMI bus is capable of transferring data, exclusive of addressing overhead, at 100 Mbps.

XML n. Acronym for eXtensible Markup Language, a condensed form of SGML (Standard Generalized Markup Language). XML lets Web developers and designers create customized tags that offer greater flexibility in organizing and presenting information than is possible with the older HTML document coding system. XML is defined as a language standard published by the W3C and supported by the industry. See also SGML.

XML attribute n. Information added to a tag to provide more information about the tag, such as <ingredient quantity= 2 units= cups >flour</ingredient>.

XML element n. Information delimited by a start tag and an end tag in an eXtensible Markup Language (XML) document. An example would be <Lastname> Davalio</LastName>.

XML entities n. Combinations of characters and symbols that replace other characters when an XML document is parsed, usually those that have other meanings in XML. For example, < represents the < symbol, which is also the opening bracket for a tag.

XML Metadata Interchange Format n. See XMI (definition 1).

XML-RPC n. Acronym for eXtensible Markup Language-Remote Procedure Call. A set of XML-based implementations that allows cross-platform and cross-programming language procedure calls over the Internet. XML-RPC permits complex data structures to be transmitted, processed, and returned between different operating systems running in different environments.

XML Schema n. A specification providing a common base for data description and validation in XML environments. XML schema replaces Document Type Definition (DTD) by defining a greater set of data types with more explicit data descriptions. XML schema has been developed as an open, vendor-neutral format to enhance information exchange and e-commerce over the Internet. It is also a standard for the description and encoding of data.

XML Schema Description Language n. See XSDL.

XML stylesheet n. Contains formatting rules that are applied to an XML file referencing the stylesheet. The standard set of rules for XML stylesheets is the Extensible Stylesheet Language (XSL). See also XSL.

XML Web services n. Units of application logic providing data and services to other applications. Applications access XML Web services via standard Web protocols and data formats such as HTTP, XML, and SOAP, independent of how each XML Web service is implemented. XML Web services combine the best aspects of component-based development and the Web and are a cornerstone of the Microsoft .NET programming model.

Xmodem n. A file transfer protocol used in asynchronous communications that transfers information in blocks of 128 bytes.

Xmodem 1K n. A version of the Xmodem file transfer protocol designed for larger, longer-distance file transfers. Xmodem 1K transmits information in 1-kilobyte (1024-byte) blocks and uses a more reliable form of error checking. See also Xmodem.

Xmodem-CRC n. An enhanced version of the Xmodem file transfer protocol that incorporates a 2-byte cyclical redundancy check (CRC) to detect transmission errors. See also CRC.

XMS n. See extended memory specification.

XMT n. Short for transmit. A signal used in serial communications.

XNS n. Acronym for Xerox Network System. A set of protocols assigned to five numbered layers (0 through 4) that form a suite designed to handle packaging and delivery of network transmissions.

XON/XOFF n. An asynchronous communications protocol in which the receiving device or computer uses special characters to control the flow of data from the transmitting device or computer. When the receiving computer cannot continue to receive data, it transmits an XOFF control character that tells the sender to stop transmitting; when transmission can resume, the computer signals the sender with an XON character. Also called: software handshake. See also handshake.

XOR n. See exclusive OR.

XOR encryption n. Short for Exclusive-OR encryption. A simple encryption scheme using the exclusive-or concept, in which a decision is based on only one of two conditions being met. Using a provided key, XOR encryption performs an exclusive-or process on each byte of data to be encrypted. Because XOR encryption is not a strong security tool used alone, it is typically used as an additional level of security for Internet transmission of sensitive information.

XPath n. An XML language for addressing items in an XML document by specifying a path through the document structure. XPath is used by XPointer and XSLT to locate and identify XML document data. XPath is also considered a query language complement to XQuery. XPath is more supported than XQuery even though there is no approved standard yet for either. See also XPointer.

XPointer n. An XML language used to locate data within an XML document based on data property descriptions, such as attributes, location, and content. XPointer references the internal structure of a document, allowing links to be made to occurrences of a word, character set, content attribute, or other element, rather than to a specific point within the document. See also XPath.

XQuery n. Short for eXtensible Query Language. Designed to be a functional query language that is broadly applicable to a variety of XML data types derived from Quilt, XPath, and XQL. Both Ipedo and Software AG implement their own versions of the W3C s proposed specification for the XQuery language. Also called: XML Query, XQL.

XSD n. Acronym for eXtensible Schema Definition. A prefix used by convention to indicate a W3C schema namespace.

XSDL n. Acronym for XML Schema Description Language. A World Wide Web Consortium (W3C) recommendation for representing XML structure. XSDL is capable of describing complex XML-based data structures, and provides options not available with Document Type Definitions (DTDs), including namespace support, XML datatypes, and improved extensibility and data type support.

X series n. A set of recommendations adopted by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), formerly the CCITT, and International Organization for Standardization (ISO) for standardizing equipment and protocols used in both public access and private computer networks. See the table.

Table X.1 Recommendations in X Series for Network Communications.

Recommendation Number	What It Covers
X.25	Interface required to connect a computer to a packet-switched network such as the Internet
X.75	Protocols for connecting two public data networks
X.200	Seven-layer set of protocols known as the ISO/OSI reference model for standardizing computer-to-computer connections
X.400	Format at the ISO/OSI application layer for e-mail messages over various network transports, including Ethernet, X.25, and TCP/IP. Gateways must be used to translate e-mail messages between the X.400 and Internet formats
X.445	Asynchronous Protocol Specification, which governs the transmission of X.400 messages over dial-up telephone lines
X.500	Protocols for client/server systems that maintain and access directories of users and resources in X.400 form
X.509	Digital certificates

XSL n. Acronym for Extensible Stylesheet Language. A World Wide Web Consortium (W3C) standard stylesheet language for XML documents. XSL determines how data in an XML document is displayed on the Web. XSL controls what data will be displayed, in what format, and in what type size and style. XSL contains two major extensions: XSL Transformations (XSLT), a language used to convert XML documents to HTML or other document types, and XSL Formatting Objects (XSL-FO), a language for specifying formatting semantics. See also XSL-FO, XSLT.

XSL-FO n. Acronym for Extensible Stylesheet Language Formatting Objects. An XML-based markup language for specifying formatting semantics. XSL-FO allows format and style information to be applied to an XML document and can be used with XSLT to produce source documents. See also XSL.

XSLT n. Acronym for Extensible Stylesheet Language Transformations. A language used in transforming an existing XML document into a restructured XML document. Formalized as a W3C Recommendation in 1999, XSLT is primarily intended for use as part of XSL. XSL describes the styling of a document in terms of XSLT transformations into an XML document. See also XML, XSL.

X terminal n. An intelligent display device, connected to an Ethernet network, that performs operations on request from client applications in an X Window System. See also Ethernet (definition 1), X Window System.

XT keyboard n. See PC/XT keyboard.

XUL n. A standards-based interface description language that provides a standard way to exchange data describing a program s user interface. XUL balances simplicity, flexibility, and ease of use with precise layout control. XUL was developed by Netscape and Mozilla and is used with XML, CSS, DOM, and HTML.

X Windows n. See X Window System.

X Window System n. A nonproprietary standardized set of display-handling routines, developed at MIT. Most often encountered on UNIX workstations, the X Window System is independent of hardware and operating system. An X Window System client calls on the server, which is located on the user s workstation, to provide a window in which the client can generate a display of text or graphics. Also called: X Windows. See also X Consortium.

X-Y display n. See vector display.

x-y matrix n. An arrangement of rows and columns with a horizontal (x) axis and a vertical (y) axis.

x-y plotter n. See plotter.

x-y-z coordinate system n. A three-dimensional system of Cartesian coordinates that includes a third (z) axis running perpendicular to the horizontal (x) and vertical (y) axes. The x-y-z coordinate system is used in computer graphics for creating models with length, breadth, and depth. See the illustration. See also Cartesian coordinates.

x-y-z coordinate system.