There is no clear definition of what constitutes a "concept album". Fiona Sturges of The Independent stated that the concept album "was originally defined as a long-player where the songs were based on one dramatic idea – but the term is subjective." A precursor to this type of album can be found in the 19th century song cycle which ran into similar difficulties in classification. The extremely broad definitions of a "concept album" could potentially encompass all soundtracks, compilations, cast recordings, greatest hits albums, tribute albums, Christmas albums, and live albums.
The most common definitions refer to an expanded approach to a rock album (as a story, play, or opus), or a project that either revolves around a specific theme or a collection of related materials. AllMusic writes, "A concept album could be a collection of songs by an individual songwriter or a particular theme — these are the concept LPs that reigned in the '50s ... the phrase 'concept album' is inextricably tied to the late 1960s, when rock & rollers began stretching the limits of their art form." Author Jim Cullen describes it as "a collection of discrete but thematically unified songs whose whole is greater than the sum of its parts ... sometimes [erroneously] assumed to be a product of the rock era." Author Roy Shuker defines concept albums and rock operas as albums that are "unified by a theme, which can be instrumental, compositional, narrative, or lyrical. ... In this form, the album changed from a collection of heterogeneous songs into a narrative work with a single theme, in which individual songs segue into one another."