[Sam Baughn]

I like the definition I once saw on RPGnet: a munchkin is the kind of roleplayer who, upon being told that the game will focus on political intrigue in fifteenth century Florence, asks to play a ninja.

In short it is a term for a player who doesn't care about the background of the game world or the enjoyment of the other players, only their own childish and tedious power fantasy. This generally involves having an ureasonably overpowered character, because that kind of person generally thinks of 'cool' and 'able to kill everyone I want to' as being closely related.