a musician in a band.

A sideman is a professional musician who is hired to perform or record with a group of which he is not formally a member. Sidemen are generally required to be adaptable to many different styles of music, and so able to fit smoothly into the group in which they are currently playing. Many sidemen are famous in their own right, however, and may be highly sought-after by bands.

A member of an ensemble who is not the leader