An indoor percussion ensemble or indoor drumline consists of the marching percussion (or battery) and front ensemble (or pit) sections of a marching band or drum corps. The only exceptions are in concert divisions (e.g. Percussion Scholastic Concert Open ) where the marching line is absent and the ensemble consists entirely of a pit. Indoor Percussion marries elements of music performance, marching, and theater; thus, the activity is often referred to as percussion theater. Although most indoor percussion ensembles are affiliated with high schools, there are also many independent groups that draw participants from a large area. Independent groups typically start rehearsing in October and groups associated with high schools start after marching band season ends in November or December, and the season culminates with national championships in April. It is sometimes called Winter Percussion due to the season in which it mostly occurs.
Definition courtesy of Wikipedia.