X.25
A packet switched protocol for network that was approved by the CCITT/ITU in 1976. It works with the 1, 2, and 3 layers of the OSI Reference Model.
X10
A protocol that communicates between transmitters and receivers by sending and receiving signals over the power line wiring. These signals involve short RF bursts which represent digital information. X10 can send commands (such as power on, power off, dim) to up to 256 nodes in a household.
X12
See EDI.
Xerox PARC
Xerox Palo Alto Research Center. The cutting edge computer think tank in Silicon Valley, CA that developed the personal computer (PC) and the graphic user interface (GUI).
XGA
eXtended Graphics Array. A standard for video cards and monitors. It has a resolution of 1,600 x 1,200 pixels. See video adapter.
x-height
The height of the lower case "x"; it is taken to be representative of the height of the lowercase letters of a typeface without their ascenders or descenders.
XHTML
eXtensible HyperText Markup Language. A variation of SGML. XHTML is an official W3C recommendation as of January 2000. XHTML is a reformulation of HTML as an XML application. XHTML 1.0 may be seen as "HTML 5.0". XHTML supports all HTML 4.0 elements and attributes but they now must conform to XML rules.
XML
eXtensible Markup Language. XML was approved by the W3C in February 1998. XML is a text-based format designed especially to transmit and store data. Whereas HTML has pre-established tags, XML has customizable tags/elements (hence the term "extensible"). Whereas HTML is concerned with the appearance of the user interface and display, XML is also concerned with the information. XML has the power of SGML but the simplicity and portability of HTML. See also my section on XML.
XML-ified
See smellified.
x-modem
Error control protocol for modems. Often used by BBSs.
XOR
A boolean operator that performs a logical exclusion on two expressions. The result of the operation is true if and only if only one of the expression is true. Here is the truth table for the XOR operator:
A  XOR  B  =  Result
0       0     0
0       1     1
1       0     1
1       1     0
XP
(1) Short for Microsoft Window XP operating system.

(2) eXtreme Programming. Aka refactoring, agile programming. A methodology for developing software that covers planning, designing, coding, and testing software. XP was initialized by Kent Beck with his 1999 book Extreme Programming Explained: Embrace Change. XP is the most famous methodology of AgileAlliance.org. It's key points include lots of customer communication, collective code ownership and paired programmers, focused prioritized tasks, code testing, short release cycles,  continual improvement. See also ExtremeProgramming.org; XProgramming.com; MartinFowler.com; Refactoring.com.

(3) Cross Platform.
XSL
eXtensible Stylesheet Language. W3C standard draft released in August 1998, it describes how XML pages are presented by an XSL processor/browser. There are a number of XSL processors available, some are listed at w3.org/XML/.
XSS
Cross-site scripting. A computer security vulnerability possible in web applications where a malicious agent can inject their own code, which if used by a user, can send some of the user's info to the malicious agent. The defense is similar to defending against SQL Injection, but when receiving values that may be outputted as HTML, simply encode the value (EG: Encode < as &lt; so the value is displayed but not "run". Note that the "XSS" abbreviation is used instead of "CSS" in order to avoid confusion with Cascading Style Sheets.
XUL
XML User-interface Language. XUL is a cross-platform language for describing the user interface of applications. XUL is made up predominantly of XML documents that describe the widgets. The resulting XUL UI is commonly modified and programmed with CSS and JavaScript. XUL was developed by Mozilla, so see also the Mozilla XUL reference.
X window system
The GUI for the UNIX OS. X was named after an earlier window system called "W". It is a window system called "X", not a system called "X Windows". X11R6 (version 11, release 6) was released in 1994/05.

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