In popular music, a concept album is an album which is "unified by a theme, which can be instrumental, compositional, narrative, or lyrical" (Shuker 2002, p.5). 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 that the members of the group or the artist have written, or have been chosen to perform or cover. 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.
In the contemporary rock era (from 1966 onwards - the point at which critics started to differentiate between "pop music" and "rock music" as a more serious form), there were, broadly speaking, two types of concept album: those that were essentially thematically-linked song cycles such as Pink Floyd's The Dark Side of the Moon which did not claim a storyline, and those that presented a narrative story that threaded the songs - such as Pink Floyd's The Wall or The Who's Tommy and Quadrophenia. Music critics of that era did not usually distinguish between the two types of concept album. An album that met either criterion was commonly referred to as a concept album. However, the distinction between the two types of concept album is important to note in respect to claims that are made as to which album may have been the first concept album in the rock era. Given this legitimate distinction, there are probably several contenders in each genre.
Another difficulty in classifying whether a given album qualifies as a concept album arises from the fact that both musicians and their listeners, through the rock era, increasingly viewed the record album as a unified art form, not simply a collection of songs. Songs on many albums may have a certain sense of cohesion even if there is no unifying lyrical theme or narrative structure. That sense of cohesion may be imposed simply by the particular lyrical or musical concerns of a composer or group of composers at the time a record was recorded. Thus, many albums that cannot genuinely be labeled concept albums in a strict sense get so designated by their fans. The albums OK Computer by Radiohead, Born to Run by Bruce Springsteen, and Rust In Peace by Megadeth are examples of this tendency. In each of these cases, there was no intention by the performers to produce an actual concept album. Some albums without any single theme or narrative structure may nonetheless have a deliberate structure in which the order in which the songs are heard expresses a particular artistic intention. Such an album, or other collection of songs, may be best viewed as a song cycle, a broader notion, with roots in classical music, that may encompass many concept albums.