Topic summary
Secret Level

Extracted from the Wikipedia article Level (video games).
Minigame
A minigame (also spelled mini-game or mini game, sometimes called a subgame or microgame) is a short video game often contained within another video game, and sometimes in application software or on a display of any form of hardware. A minigame contains different gameplay elements than the main game, may be optional, and is often smaller or more simplistic than the game in which it is contained. Minigames are sometimes also offered separately for free to promote the main game. Some minigames can also be bonus stages or secret levels. They are distinguishable from levels in that a level is an environment bound to a set of mechanics and rules that all defines all other normal levels in a game, whereas a minigame can use different rules and playstyles but may not necessarily be set in a different environment.