What is a jam session?

A jam session is a musical act where musicians gather and play (or "jam") without extensive preparation or arrangements.

The origin of the term jam in this context can be traced back to the 1920s. According to the Online Dictionary of Etymology, the term originally appeared ca. 1929, referring to a "short, free improvised passage performed by the whole band". The derivation of this usage is obscure, but like other novel terms that came into English through jazz music — such as the terms "hip", "hep" and "hepcat" — it is possible that it ultimately derives from the West African Wolof language.

The word 'jam' can be more loosely used to refer to any particularly inspired or improvisational part of a musical performance, especially in rock and jazz music. Jam sessions, however, are generally for the benefit of the performers and not part of a public performance.

Jam sessions are often used to develop new material, 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 chart suggested by one participant, or may be wholly improvisational. Jam sessions can range from very loose gatherings of amateurs to sophisticated improvised recording sessions intended to be edited and released to the public.

The New York jazz scene during World War II was famous for its after-hours jam sessions. One of the most famous was the regular after-hours jam at Minton's Playhouse in New York City that ran in the 1940s and early 1950s. The Minton's jams were a fertile meeting place and proving ground for both established soloists like Ben Webster and Lester Young, and the younger jazz musicians who would soon become leading exponents of the bebop movement, including Thelonious Monk (Minton's house pianist), Charlie Parker, and Dizzy Gillespie. The Minton's jams were legendary for their highly competitive "cutting contests", in which soloists would try to keep up with the house band and outdo each other in improvisation skill. In recent years, some restaurants, bars, and clubs have been featuring jam session nights, where either bands come in and play, or soloists play with a band who has already played at the location. "Open mic nights" are common also, which usually feature only a singer, or piano and vocal together.