The underlying logic of the game, the relationships between inputs and outputs, and the logical structure of success or progress and failure. Systems are the mechanical guts of a game, once all the metaphors are stripped away.

Tetris, no matter what fan-fiction you subscribe to, is a game where most of the system is fed, unfiltered, directly to the player. Fit shapes into holes, there are this many shapes, here’s some music, I guess.”