Definitions for "Jam Session"
an event where a bunch of musicians play music they all know, or make up on the spot without rehearsal
An informal gathering and performance of musicians, stressing improvisation.
Jam Session is a 1942 short film which shows Duke Ellington and his orchestra performing C Jam Blues.
A get-together of jazz players
an impromptu jazz concert
informal playing session by a pop or jazz group
fundraising by choral society members, their wives, aunts and grannies
a group of improvisers at work.