Generally (but not always) a sacred composition in multiple parts for voices and instruments. The most famous are the more than two hundred by J.S. Bach, who wrote one every WEEK for the duration of a particularly demanding gig. At their best, sublime, memorable, and utterly gorgeous; at their worst (i.e. a lot of the non-Bach) some of the dullest stuff ever composed. Most of this latter type is rarely heard nowadays.
