In popular music, a concept album is an album which is "unified by a theme, which can be instrumental, compositional, narrative, or lyrical". Most often they are pre-planned (conceived) and with all songs contributing to a single overall theme or unified story, this plan or story being the concept. This is in contrast to the standard practice of an artist or group releasing an album consisting of a number of unconnected songs performed by the artist. Given that the suggestion of something as vague as an overall mood often tags a work as being a concept album, a precise definition of the term proves problematic.