Jam session
A jam session is a musical event, process, or activity where musicians play (i.e. "jam") by improvising without extensive preparation or predefined arrangements. Jam sessions are often used by musicians to develop new material (music), find suitable arrangements, or simply as a social gathering and communal practice session. Jam sessions may be based upon existing songs or forms, may be loosely based on an agreed chord progression or chart suggested by one participant, or may be wholly improvisational. Jam sessions can range from very loose gatherings of amateurs to evenings where a jam session coordinator acts as a "gatekeeper" to ensure that only appropriate-level performers take the stage, to sophisticated improvised recording sessions by professionals which are intended to be edited and released to the public.
Jazz
The phrase "jam session" came about in the 1920s when white and black musicians would congregate after their regular paying gigs, to play the jazz they couldn't in the "Paul Whiteman" style bands. Bing Crosby would often attend these sessions, and the musicians would say he was "jammin' the beat", since he would clap on the one and the three. Thus these sessions became known as "jam sessions".