It's defined as, "An overzealous or obsessive fan of a particular celebrity." It can be used as a verb, too, as in, "I k**w Eminem is a rap god, but I really stan for Dido."
The term originated from Eminem's song, Stan, about a dangerously obsessed fan. It's since been softened and incorporated in the fandom vernacular to describe a particularly devoted fan of an artist. Members of Rihanna's Navy, the Bey Hive, Arianators, Directioners all self-identify as stans.* Read more...