Historical Rules · 1 February 2010
Rule 712.1a
Official source text from the 1 February 2010 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.