A session musician is a musician hired to perform in a recording session or a live performance. The term sideman is also used in the case of live performances, such as accompanying a recording artist on a tour. Session musicians are usually not permanent or official members of a musical ensemble or band.
Session musician Hal Blaine (pictured in 1995) is widely regarded as one of the most prolific drummers in rock and roll history, having "certainly played on more hit records than any drummer in the rock era".
A sideman is a professional musician who is hired to perform live with a solo artist, or with a group in which they are not a regular band member. The term is usually used to describe musicians that play with jazz or rock artists, whether solo or a group.
David Lindley (1980) playing the pedal steel guitar
Bernard Fowler, backup vocalist for The Rolling Stones