jam session

an informal gathering of musicians, esp. jazz musicians, to play improvised music or to improvise together, usually on tunes they all know
jam session

noun
- An informal gathering of musicians to play improvised or unrehearsed music.
- Informal An impromptu discussion.
jam-session

Noun
(plural jam sessions)
- (music) An informal gathering of musicians to play music, especially improvised jazz or a similar genre.