A jazz term. It's someone who isn't the leader of the group, whose name doesn't get the large, boldface type on the flyers, ads, and LP/CD covers. Sometimes it's a sideman (sideperson?) who provides the most musically interesting statements in the group (e.g. a very young Wynton Marsalis in Art Blakey's Jazz Messengers, or saxophonists like John Coltrane, Cannonball Adderley, and Wayne Shorter in Miles Davis' quintets and sextets); sometimes not.