(1) The exit button on an application window. In most cases, clicking X closes the entire application and all open windows that pertain to it. In some applications, such as the Firefox browser, clicking X closes only that window, not additional windows opened from the original instance.

(2) 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) A performance rating for CD-ROM and DVD drives, based on rotation speed (RPM) of the platter. Each "x" means "times the speed of the first drive." 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.

The programming language in the Axapta development environment. See Microsoft Dynamics AX.