A term coined by Wario Ware Inc.. The microgame genre consist of games made up of very short "microgames" games put together as a game. A microgame is a lot like a minigame but shorter. In later Wario Ware games the microgames became longer, but the one continuing factor between each game in the series, was that what they called 'microgames' made up most of the gameplay. So, essentially, a microgame is different from a minigame in that a minigame is a diversion from the main game, whereas a microgames are usually shorter and more plentiful and make up most of the main game.
Some popular Microgame games: