a group of people (usually around 15 to 30) that perform 5-10 minutes of music indoors after marching season has ended. ages of people involved depend on the category (scholastic-highschoolers, independant-any age up to age limit, open-mostly college aged up to age limit, world-mostly college aged up to age limit).
ensembles usually are split into front ensemble (marimba, xylophone, vibraphone, synth, toys) and battery (snare, tenor, bass drums, cymbals) or in the case of concert ensembles, that have just front ensemble with no marching. the music difficulty depends on the categories (or a mixture: i.e. independant open, scholastic A) and usually follows a theme or idea shown through the music.
indoor percussion groups are insane, how do they play all those notes?!