A compilation album comprises tracks, which may be previously released or unreleased, usually from several separate recordings by either one performer or by several performers. If the recordings are from one artist, then generally the tracks were not originally intended for release together as a single work, but may be collected together as a greatest hits album or box set. If the recordings are from several artists, there may be a theme, topic, time period, or genre which links the tracks, or they may have been intended for release as a single work—such as a tribute album. When the tracks are by the same recording artist, the album may be referred to as a retrospective album or an anthology.