(1) (eXit) To exit, close or delete. See X button.
(2) (heX) In programming, a symbol used to identify a hexadecimal number. For example, "0x0A" and "\x0A" specify the hex number 0A. See also X Window and MDI-X port.
(3) (times the original) A performance rating for optical drives, based on rotation speed (RPM) of the platter. Each "x" means "times the speed of the first specification," thus 10x means 10 times as fast as the original drive. See CD-ROM drives and DVD drives.
(4) (x-) The prefix used to describe a MIME type. For example, x-pdf and x-gzip refer to PDF and Gnu ZIP file formats, respectively. See MIME type.
(5) A prefix for cross; for example, x-platform means "cross-platform." See cross platform.
(6) (Mac OS X) X is the Roman Numeral for 10; thus OS X means "OS 10." See Mac OS X.
(7) (eXperience) See UX designer and user experience.