A subgame refers to a smaller part of a larger game. It is like a game within a game. Players in a subgame focus on a specific situation or decision, without worrying about the overall game. Full definition
There's enough weird subgames in enough Japanese games that I suspect it's just that Japanese people want a bunch of shit to do in their games. (gameological.com)
Whenever you started enjoying Kirby games, be it his first Game Boy jaunt through Dream Land, combining powers on the N64 to collect Crystal Shards, or saving an entire galaxy and getting addicted to subgames on the SNES; there's been something for everyone and everything for someone. (nintendowire.com)
The Fallout games could never shake Bethesda's penchant for fantasy quests and character builds and elaborate inventory subgames and dialogue trees and hearty faithful sidekicks. (quartertothree.com)