Nightcap: The Unreleased Masters 1973–1991 (1993) is a double compilation album by Jethro Tull released on 22 November 1993. It contains much of the band's previously unreleased material.
The first disc contains material recorded in August 1972, much of which was re-recorded and re-arranged for the band's sixth album, A Passion Play (the lyrics of "Critique Oblique" and "Scenario" actually refer to the "passion play" in question). The songs "Scenario", "Audition" and "No Rehearsal" initially appeared on the 20 Years of Jethro Tull box set as a single track entitled "The Chateau d'Isaster Tapes". This humorous title is also used on Nightcap as the title of the entire first disc. The material on the first disc was mixed and arranged as the aforementioned three tracks were on the 20 Years box set with numerous flute overdubs by Ian Anderson, but excludes the songs "Big Top" and "Sailor". All "Chateau d'Isaster" material was included as part of the 2014 re-release of A Passion Play, without any overdubs.
The second disc contains unreleased material recorded between 1974 and 1991, in particular extra songs from the sessions for The Broadsword and the Beast. Most, but not all, of these songs also appear as bonus tracks on remastered versions of Jethro Tull's 1970's albums.
The album was produced in limited quantities with proceeds going to charity.