What a game-master is for, according to 30+ games

Martin sez, “Treasure Tables has taken 30+ role-playing games and written down their definitions of what the game-master does.”

Advanced Dungeons & Dragons – Dungeon Master’s Guide, 2nd Edition (TSR, 1989)
Being a good Dungeon Master involves a lot more than knowing the rules. It calls for quick wit, theatrical flair, and a good sense of dramatic timing, among other things. (Page 7 of 192)

All Flesh Must be Eaten – Core book, 1st Edition (Eden Studios, 1999)
To enforce the rules and provide a coherent setting, one of the participants assumes the role of Zombie Master (called Game Master, Chronicler or Referee in other contexts). (Page 20 of 231)

Amber – Core book (Phage Press, 1993)
This is the person who controls the “world” and runs the game. All the non-player characters (NPCs), including guards, innocents, and villains are controlled by the GM. The GMs [sic] control even extends to things like weather, cross-universe politics and natural disasters. (Page 9 of 256)

Bunnies & Burrows – Core book (Fantasy Games Unlimited, 1976)

Rather, there is a Gamemaster (GM) that oversees the game, designs the playing area, is expected to modify the rules given herein to suit his or her fancy, and is the only omniscient participant in the game. (Page 1 of 74)

Link (Thanks, Martin!)