Initialism of Extreme Programming.
Initialism of Experience Point.
Abbreviation of extreme prix; xPrix.; from being an extreme grand prix.
Initialism of 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.)
Ellipsis of Windows XP
eXperimental, Pursuit, fighter (prefix)