XP is a slang term which is often used for describing computer OS, Microsoft Windows XP.
The abbreviation XP is used in games for eXPerience in MMORPGs or RPGs. To be specific, the term XP is used when the level of the game increase or the player defeats his opponent on the same level.
It is also being used as an emoticon, XP or xP, meaning tongue sticking out. We also have Athlon XP 3100+ AMD series processors, which is the 7th gen series.