From Longman Dictionary of Contemporary Englishjam sessionˈjam ˌsession (also jam) noun [countable] APMan occasion when jazz or rock musicians play music together in an informal way
Examples from the Corpus
jam session• Then on Sunday night all styles will merge in a monumental jam session.• Baker was first heard no record in a public jam session of 1941 and quickly established an unbeatable reputation in London clubs. 