In chess, a "miniature" is a term generally used to describe a decisive (i.e. non-drawn) chess game that lasts no more than 20 moves in total (or perhaps a bit more, but never more than 25 moves). There is no official definition of the term, hence the small amount of wiggle room with the cutoff.
Another aspect of the "miniature" concept is that it has to be a pretty good game. In other words, a game where one player simply blunders an important piece for no good reason and loses quickly or simply resigns would pretty much never merit the name "miniature." These are good, exciting games where one player wins a decisive advantage with brilliant play, often involving clever traps or sacrificing large amounts of material in exchange for an unstoppable attack. Most games deemed "miniatures" also end in actual checkmates, rather than resignations, or a resignation just short of a forced checkmate, rather than one player simply being a bit worse and deciding to resign.
In sum, the word "miniature" is generally reserved for brilliant attacking games against a capable opponent that result in stunning, sudden victories.