Term I coined when describing the role-playing game Rifts (however it could probably be applied to many FRPGs).
The reasoning is that each new Rifts book published contains rules that allow munchkins to create more powerful characters than any of the previous books. Because of this munchkins are compelled to "collect" the latest books in order to be the best (worst?) munchkins they can be.
See also: collectible cert game and collectible card game.