Abbreviation for Game Master, a general title for any person who runs a role-playing game, particularly on a regular basis.

A good GM should care about his or her game, make an engaging setting and underlying plot (ideally with significant artistic value), not expect or force linear progression, be prepared when the players do something entirely unexpected, consider what the players want and what their characters want, make their NPCs more than just mouthpieces, be careful to balance the need to have NPCs stay in character with the need for them not to steal the spotlight, keep the players organized and coming regularly, and perhaps most importantly, ALWAYS SHOW UP!

*pant, pant* Sorry, I just tend to run into GMs who skip out on their own games. They seem not to realize that while being the GM is fun, it's also a responsibility. It's highly aggravating.