Historical Rules · 11 November 2016
Rule 718.1a
Official source text from the 11 November 2016 Rules release
A “subgame” is a completely separate Magic game created by an effect. Essentially, it’s a game within a game. The “main game” is the game in which the spell or ability that created the subgame was cast or activated. The main game is temporarily discontinued while the subgame is in progress. It resumes when the subgame ends.