X:

XML - extensible markup language: XML is a pared-down version of SGML, which is designed especially for Web documents.
XML style sheets: The CS Help compiler provides XML style sheets that allows a representation of the help file to be viewed in an XML enabled browser.
XOFF: Flow control character used to signal to a transmitter that it should suspend data transmission. The character is configurable, with the industry standard being decimal 19.; See also: high water mark
XON: Flow control character used to signal to a transmitter that it should resume data transmission. The character is configurable, with the industry standard being decimal 17.; See also: low water mark