In the late 1960s, the term supergroup was coined to describe music groups comprising members who had already achieved fame or respect in other groups or as individual artists. The term took its name from the 1968 album Super Session with Al Kooper, Mike Bloomfield, and Stephen Stills. The coalition of Crosby, Stills and Nash (later Crosby, Stills, Nash and Young) is another early example. Led Zeppelin can also be termed as an early supergroup, as it was originally meant to act as a replacement for The Yardbirds, and the members were chosen by Jimmy Page (already a much sought after guitarist) by virtue of their already impressive catalogue of work (except for Robert Plant, who was chosen because of his very distinct voice).
Supergroups on Wikipedia.