A jam band is a musical group whose concerts and live albums substantially feature improvisational "jamming." Typically, jam bands will play variations of pre-existing songs, extending them to improvise over chord patterns or rhythmic grooves. Jam bands are known for having a very fluid structure, playing long sets of music which often cross genre boundaries, varying their nightly setlists, and segueing from one song into another without a break.

The jam-band musical style, spawned from the psychedelic rock movement of the 1960s, was a feature of nationally famed groups such as the Grateful Dead and The Allman Brothers Band, whose regular touring schedules continued into the 1990s. The style influenced a new wave of jam bands who toured the United States with jam band-style concerts in the late 1980s and early '90s, such as Phish, Blues Traveler, Widespread Panic, Dave Matthews Band, The String Cheese Incident, moe., and Col. Bruce Hampton and the Aquarium Rescue Unit. The jam-band movement gained mainstream exposure in the US in the early 1990s with the rise of Phish and the Dave Matthews Band as major touring acts and the dissolution of the Grateful Dead following Jerry Garcia's death in 1995.

Jam-band artists often perform a wide variety of genres. While the Grateful Dead is categorized as psychedelic rock, by the 1990s the term "jam band" was applied to acts that incorporated genres such as blues, country music, contemporary folk music, funk, progressive rock, world music, jazz fusion, Southern rock, alternative rock, acid jazz, bluegrass, folk rock and electronic music into their sound. Although the term has been used to describe cross-genre and improvisational artists, it retains an affinity to the fan cultures of the Grateful Dead or Phish.

A feature of the jam-band scene is fan recording of live concerts. While the mainstream music industry often views fan taping as "illegal bootlegging", jam bands often allow their fans to make tapes or recordings of their live shows. Fans trade recordings and collect recordings of different live shows, because improvisational jam bands play their songs differently at each performance.