X /X/ n.

1. Used in various speech and writing contexts (also in lowercase) in roughly its algebraic sense of `unknown within a set defined by context' (compare N). Thus, the abbreviation 680x0 stands for 68000, 68010, 68020, 68030, or 68040, and 80x86 stands for 80186, 80286, 80386, 80486, 80586 or 80686 (note that a Unix hacker might write these as 680[0-6]0 and 80[1-6]86 or 680?0 and 80?86 respectively; see glob). 2. [after the name of an earlier window system called `W'] An over-sized, over-featured, over-engineered and incredibly over-complicated window system developed at MIT and widely used on Unix systems.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.