- (computing) Initialism of extreme programming.
- (role-playing games) Initialism of experience points.
- (computing) Windows XP
X is a variable which can stand for letters representing parts of speech such as: N (noun), V (verb), P (preposition), etc.
- (grammar, X-bar theory) "X Phrase": a phrase which consists of either: (1) an optional specifier and an X-bar phrase, or (2) a conjunction sandwiched between two other XPs. The X is a "pro-letter" which can be substituted by letters such as N for noun, V for verb, P for preposition, I for inflectional, etc.