The right definition of the word "GM" is Game Moderator.
Noobs often confuse this with Game Master, even though everyone could be a game master.
Player 1: "Do you know what GM means?"
Player 2: "Game Master, Doh!"
Player 1: "Wrong noob!, it means Game Moderator."
Player A: "Stop harassing me or I will call a Game Master"
Player B: "It's Game Moderator, you noob!"