Formal:
* Piece: This is a very general term that can refer to any musical composition, short or long.
* Movement: Often used for a section of a larger work like a symphony or concerto.
* Interlude: A short piece played between other pieces, often in a play or opera.
* Entr'acte: Similar to interlude, specifically used between acts of a play or opera.
* Prelude: A short piece played before a longer work.
* Postlude: A short piece played after a longer work.
Informal:
* Ditty: A simple, catchy song.
* Tune: A short, memorable melody.
* Snippet: A brief excerpt from a longer piece.
* Jingle: A short, catchy musical phrase, often used for advertising.
* Fanfare: A short, loud, and festive piece of music.
* Riff: A short, repeated musical phrase.
More specific:
* Nocturne: A short, dreamy piece of music, often played at night.
* Scherzo: A lively, playful, and often humorous musical composition.
* Caprice: A short, free-form piece of music.
* Barcarolle: A piece of music that evokes the rocking motion of a gondola.
* Waltz: A piece of music in a three-quarter time signature, often associated with ballroom dancing.
The best word to use will depend on the specific context and the style of the music.