The just added (and very comprehensive) Glossary lists three interrelated definitions for the term Leader. These include "Leader", defined as "usually...the artist or artists who direct the performers in a recording session," Issue Leader, "The name of the performing artist or artists who lead the performances on the issue Tracks", and Alternate Leader, 'The name of the listed album leader only if different than the individual, or any of the individuals, defined as leaders of the pertinent sessions." This seems to correspond well to reality.
In Musician's Union contracts leader is used to define the person responsible for the performance (mostly for financial purposes). On a typical gig the leader is the person who hires the musicians, calls the music, pays the band, etc. Recording sessions are much more complex, but most jazz sessions have a specific leader with responsibility for the music.
An idea of the potential complications in defining a leader can be seen the the Ornette Coleman Hillcrest Club recordings. J-DISC has been adding some early Ornette to the database, and that included the eight tracks (released in two groups of four each on two LPs) recorded at the Hillcrest Club in Los Angeles in 1958 (October 1958 (a) and October 1958 (b) in the database). From an historical point of view the leader on these recordings is clearly pianist Paul Bley. Bley was leading a quartet with Dave Pike, vibes; Charlie Haden, bass and Lenny McBrowne (later Billy Higgins) drums, on a steady gig at the Hillcrest. Haden had just begun rehearsing with Coleman and brought him and Don Cherry in to the Hillcrest to sit in one night. The effect on Bley seems to have been dramatic. He's quoted (in John Litweiler's Ornette Coleman: The Harmolodic Life, from a 1979 Coda interview with Bill Smith) as recalling that he and Haden went out behind the club and had a discussion: "We said, 'Look, we have been working in this club for a long time and most probably could stay here as long as we wanted. If we fire Dave Pike and hire Don and Ornette we won't last the week. We'll be lucky to last the night. What shall we do?" And we looked at each other and said, 'Fire Dave Pike!" (They actually lasted six weeks or so).
All of these details, particularly Bley's power to choose his sidemen, indicate that he was the leader. Yet creatively the dominant force obviously was Coleman. There's a sense in the quote (and in the subsequent treatment of the music) that Bley recognized what Ornette was about to do to the music and wanted to give him an opportunity to explore (and to be a part of it). Bley recorded the music that was later released. The music was largely Ornette's--5 of the 8 titles are his originals. In retrospect it looks like the classic Ornette Coleman Quartet with Bley tagging along, but in reality it seems to have been players with an affinity for what both were doing. Bley produced one of the two original releases (on IAI) and must have been the source for the other; neither lists a leader per se. Bley's name comes first in the list of performers, but only the Inner City release shows Bley's name on the spine of the LP. And the IAI release is called "Coleman Classics". Discography is a history of the music, and not a history of the music business, so I think it makes sense to see these as Ornette Coleman sessions and to list Coleman as leader or at least co-leader with Bley. --David Wild