A mind is a disembodied actor controlled by the game. Minds are often used as invisible enemies in single player games (such as single player Chess), but also sometimes as allies. Like a disembodied player, the mind controls characters and objects in the game but is never directly seen.
Though a mind is a part of a game, it is not fully aware of the entire game (at least, not from the player’s perspective). It appears to play fair, make mistakes, follow choice-making heuristics and sometimes even has slightly different personality traits from other minds in the game.
The term ‘mind’ comes from the Culture novels of Iain M Banks, and is used here because it is neater than ‘artificial intelligence’ or ‘simulated sentience’ etc.