/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 or 80486 (note that a UNIX hacker might write these as 680[0-4]0 and 80[1-4]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.