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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.

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 or 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.

As the instrumental proficiency of pop and rock musicians improved in the Sixties and early Seventies, onstage jamming also became a regular feature of rock music; bands such as Cream, The Jimi Hendrix Experience, The Grateful Dead and the Allman Brothers Band would feature live pieces easily over fifteen minutes in length.

More recently, the Jam Band has become a genre unto itself; performances by groups including Moe., Umphreys Mcgee, Phish, and Widespread Panic feature long feature extended improvisational sessions. Other bands, such as the Red Hot Chili Peppers also regularly perform live jam sessions as part of an encore.

Some notable recorded jams in the rock idiom:

• The bonus CD of the 25th anniversary of the album Layla and Other Assorted Love Songs by Eric Clapton's early 70s band, Derek & The Dominos includes a number of long improvised jams between members of the group and other musicians, such as The Allman Brothers Band following the historic first meeting between the two groups earlier that day. As a result of this jam, guitarist Duane Allman was invited to join the Dominos after having recorded only three songs, and he made a major contribution to the resulting LP.
• The farewell Last Waltz concerts led by The Band in 1976 included two late-night jam sessions featuring Neil Young, Ron Wood, Eric Clapton, Ringo Starr and others along with members of The Band. These performances were not included in the film or original recordings of the concert officially released for the first time as part of a 2002 box set.
• The extended track "Apple Jam", which appears on George Harrison's 1970 solo album All Things Must Pass and features most of the session musicians who contributed to the LP.
• Nirvana's improvised hidden track "Gallons of Rubbing Alcohol Flow Through the Strip" from the album In Utero (1993), and 'semi-improvised' hidden track from Nevermind (1991).
• Liquid Tension Experiment's "Three Minute Warning" from the album Liquid Tension Experiment was a 28 minute long improvised jam, the result was so unexpectedly fluent that the members decided to include it on the release.
• Bluesbreaker by Brian May and Friends recorded during the Star Fleet Project and featuring Eddie Van Halen, Phil Chen, Alan Gratzer and Fred Mandel.

Bluegrass music also features a tradition of jamming. Bluegrass jams happen in the parking lots and campgrounds of bluegrass festivals, in music stores, bars and restaurants and on stages. Bluegrass jams tend to be segregated by the skill level of the players. Slow jams for beginners provide an entry point. Open bluegrass jams are open to all comers, however, the players in an open jam will expect a certain level of proficiency. The abilities to hear chord progressions and keep time are prerequisite; the ability to play improvised leads that contain at least a suggestion of the melody is desired. Jams that require advanced musical proficiency are generally private, by-invitation events.

• Jam band
• Jamming (dance)
• Free improvisation
• Freestyle rap
• Bluegrass music
• Finding Bluegrass (and acoustic folk) jam sessions
• Session, including Session musician and Irish traditional music session (this is the folk equivalent of a jam session)