jam session

improvised musical session (rock, jazz, etc.)

A jam session is a musical act where musicians gather and play (or "jam") without extensive preparation or predefined 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.
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Jam Session is a 1942 short film which shows Duke Ellington and his orchestra performing C Jam Blues.In 2001, the United States Library of Congress deemed the 3-minute, black-and-white film "culturally, historically, or aesthetically significant" and selected it for preservation in the National Film Registry.
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Noun
1. an impromptu jazz concert
(hypernym) musical performance

(new spell.=Jamsession) jam session, improvised musical session

informal practice session for musicians, jam After the symphony rehearsal, a few of us had a jam session.