One source for 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 could not play in the "Paul Whiteman" style bands they played in. When Bing Crosby would attend these sessions, 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". Mezz Mezzrow also gives this more detailed and self-referential description, based on his experience at the jazz speakeasy known as the Three Deuces:
I think the term 'jam session' originated right in that cellar. Long before that, of course, the colored boys used to get together and play for kicks, but those were mostly private sessions, strictly for professional musicians, and the idea was usually to try to cut each other, each one trying to outdo the others and prove himself best. Those impromptu concerts of theirs were generally known as 'cuttin' contests.' Our idea…was to play together, to make the improvisation really collective… Down in that basement concert hall, somebody was always yelling over to me, 'Hey Jelly, what you gonna do?'—they gave me that nickname, or sometimes called me Roll, because I always wanted to play Clarence Williams' '(I Ain't Gonna Give Nobody None O' This) Jelly Roll'—and almost every time I'd cap them with, 'Jelly's gonna jam some now,' just as a kind of play on words. We always used the word 'session' a lot, and I think the expression 'jam session' grew up out of this playful yelling back and forth.
The New York 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 jam sessions at Minton's 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), saxophone player Charlie Parker, and trumpeter Dizzy Gillespie. The Minton's jams had competitive "cutting contests", in which soloists would try to keep up with the house band and outdo each other in improvisational skill.
Influenced by jazz, Cuban music saw the emergence of improvised jam sessions during the filin movement of the 1940s, where boleros, sones and other song types were performed in an extended form called descarga. During the 1950s these descargas became the basis of a new genre of improvised jams based on the son montuno with notable jazz influences pioneered by the likes of Julio Gutiérrez and Cachao. During the 1960s, descargas played an important role in the development of salsa, especially the salsa dura style.
As the instrumental proficiency of pop and rock musicians improved in the 1960s and early 1970s, onstage jamming—free improvisation—also became a regular feature of rock music; bands such as Pink Floyd, Cream, the Jimi Hendrix Experience, Deep Purple, The Who, the Grateful Dead, AC/DC, Led Zeppelin, Santana, King Crimson and the Allman Brothers Band would feature live improvised performances that could last anywhere between 10 and 20 minutes. However, they can be shorter on the recorded version.