X12
A electronic data interchange standard (EDI) standard from the American National Standards Institute (ANSI). See EDI and ANSI.

X.21
An International Telecommunication Union (ITU) specification that describes the interface used in the ITU X.25 packet-switching protocol, and in some types of circuit-switched data transmissions. See X.25.

X.25
A wide-area network protocol for system and network interconnection. Developed in the early 1960s, was the first packet data network standard. Although it is fairly primitive by contemporary standards, X.25 remains heavily used.

X.28
An International Telecommunication Union (ITU) recommendation. X.28 defines the exchange of commands and responses in an X.25 network between a packet assembler/disassembler (PAD) and the asynchronous terminals attached to it. See ITU and PAD.

X.3
International Telecommunication Union (ITU) specification for the packet assembler/disassembler (PAD) facility in a public data network. See ITU and PAD.

X.400
An International Telecommunication Union recommendation covering the handling of messages, such as storing and forwarding e-mail, over digital circuits. X.400 enables e-mail messages to be transported between different mail systems.

X.500
The International Telecommunication Union (ITU) standard for directory services. The X.500 recommendation covers the implementation of addressing databases for devices attached to a network. See ITU and directory service.

X.509
The certification authority standard administered by the International Telecommunication Union (ITU). See ITU and certification authority.

X9.9
An American National Standards Institute (ANSI) standard covering message authentication in banking-industry electronic data interchange (EDI) transactions. See ANSI and EDI.

xCBL (XML Common Business Library)
The specification for the XML-centric transactional infrastructure used in Commerce One's e-marketplace platform. xCBL was originally developed by Veo Systems, an XML developer acquired by Commerce One in January 1999. xCBL is more ambitious than Ariba's cXML, in that its usage is not designed to be restricted to the marketplace environment.

xDSL
Any of several variants of digital subscriber line (DSL). See DSL, ADSL, HDSL, SDSL and VDSL.

Xenix
A multiuser operating system developed by Microsoft. A subset of Unix, it is no longer sold.

Xeon
Intel's high-performance workstation processor, which is based on the same "P6" core as the Pentium II and Pentium III chips.

XGA (Extended Graphics Array)
A high-resolution video display mode that provides screen pixel resolution of 1,024 by 768 in 256 colors, or 640 by 480 in high (16-bit) color.

XHTML (Extensible Hypertext Markup Language)
A World Wide Web Consortium (W3C) recommendation that defines Hypertext Markup Language (HTML) 4.0 in Extensible Markup Language (XML) syntax, and as an XML application. See HTML, XML and W3C.

XLink (XML Linking Language)
The language used to create and manage links among content components written in Extensible Markup Language (XML). XLink uses XML syntax to create constructs that can be inserted into XML documents to create links of varying complexity — from the simple, unidirectional hyperlinks of Hypertext Markup Language to more sophisticated, multidirectional links. Specifications for the language are defined in the World Wide Web Consortium's proposed XLink Recommendation. See XML.

XMI (XML Metadata Interchange)
An Extensible Markup Language (XML) grammar for exchanging metadata between repositories. See XML and metadata.

XML (Extensible Markup Language)
A metalanguage approved as a World Wide Web Consortium (W3C) recommendation in February 1998. A simplified version of Standard Generalized Markup Language (SGML), XML captures SGML's key advantages (such as extensibility) without its more obscure features. Because it is a metalanguage (a language to define languages), it intrinsically offers Hypertext Markup Language (HTML) capabilities and can be used to create HTML documents. A family of XML-related standards (formally called "recommendations") has been under development by the W3C since 1997. These include XML Linking Language (XLink), XML Path Language (XPath), XML Pointer Language (XPointer), Extensible Stylesheet Language (XSL) and XSL Transformations (XSLT). Together, they form a critical foundation for today's Web-based computing and e-commerce infrastructures. (See XLink, XPath, XPointer, XSL and XSLT.)

XML Namespace
An XML specification for qualifying the tag names used in XML documents by associating them with their source.

Xmodem
An 8-bit, public-domain error-checking protocol developed in the late 1970s. The file transfer protocol uses a 128-byte data block and cyclic redundancy check (CRC) or checksum error checking. See CRC.

X/Open
Originally, an international consortium of computer vendors working to advance the deployment of Unix. X/Open merged with the Open Software Foundation to form the Open Group in 1996, and the term "X/Open" is now used as an Open Group branding program.

XPath (XML Path Language)
A language used to create addresses mapping the various parts of Extensible Markup Language (XML) document. XPath gets its name from its use of a path notation (as used in Internet URLs) for navigating through the hierarchical structure of an XML document. XPath addresses are used by Extensible Stylesheet Language Transformations (XSLT) and XML Pointer Language (XPointer). Specifications for the language are defined in the World Wide Web Consortium's XPath Recommendation. See XML, XPointer and XSLT.

XPointer (XML Pointer Language)
XPointer provides the method for referencing Extensible Markup Language (XML) content. It is used to reference content components or anchors, the constructs that support addressing in the internal structures of XML documents. In particular, it provides for specific reference to elements, character strings and other parts of XML documents, whether or not they bear an explicit ID attribute. Specifications for the language are defined in the World Wide Web Consortium's XPointer Recommendation. See XML.

XRC (Extended Remote Copy)
An asynchronous remote-copy capability that works with IBM's MVS operating system.

XSL (Extensible Stylesheet Language)
XSL associates presentation characteristics (such as layout) with the markup used in Extensible Markup Language (XML). One of the XML family of languages developed by the World Wide Web Consortium, XSL is used to create XML "stylesheets," which describe how XML documents are presented. XSL provides independent control of presentation from content and can describe output of the same content to different formats (e.g., audio or print). XSL is made up of three components:

XSLT (Extensible Stylesheet Language Transformations)
A component of Extensible Stylesheet Language (see XSL), XSLT controls views of Extensible Markup Language (XML) documents and the ordering of XML elements. It is used to create new content structures from existing structures or subsets, based on interest, access privileges or security, and it transforms XML structure to Hypertext Markup Language. Specifications for the language are defined in the World Wide Web Consortium's XSLT Recommendation, published in November 1999.

xSP
A catch-all term for the emerging category of vendors — such as application service providers (ASPs) and business service providers (BSPs) — that use a network-based, "pay as you go" service delivery model. See ASP and BSP.

X terminal
A workstation operating under X-Windows.

X-Windows
The software system written for managing windows under Unix. A graphics architecture, application programming interface and prototype implementation developed at the Massachusetts Institute of Technology, X-Windows defines a client/server relationship between the application program and the workstation. It is not, however, a complete graphical user interface, but rather the basis on which one can be built.