Minigame
A minigame, sometimes called a subgame is a short video game contained within another video game. Usually the minigame is smaller or more simplistic than the game in which it is contained.
Minigames occur either as gameplay features, or as Easter eggs. In the latter case, they are often called "secret games".
Some games, such as Wario Ware, Inc., or some Cinemaware titles like Defender of the Crown are made up of many minigames strung together into one video game. Similar games such as the Mario Party series are considered party games, games specifically developed for multiplayer amongst players. Usually the minigames involve doing something faster or collecting more of something than other players to win.
An example of a minigame is Arcomage, contained within Might and Magic VII. It involves building up a tower whilst trying to destroy the computer opponent's tower. It is a simple but very addictive game that can be played at the inns around the Might and Magic VII game-world, the same way Shenmue II has arcades of early Sega games available to play.
A particularly notable subgame is in Maniac Mansion 2: Day of the Tentacle, the whole original Maniac Mansion game can be played on a computer that is encountered.