Concept musical
A 'concept musical' is a musical where the show's metaphor or statement is more important than the actual narrative. Concept musicals have their roots in the plays with music Bertolt Brecht wrote with Kurt Weill and other composers. They were introduced to Broadway in the 1940s, but first flourished in the late 1960s, and became more popular in the 1970s, a trend that was set off by Hair.
There are three major contenders for the title of first concept (even though the term "concept musical" hadn't been coined when any of them played): Lady in the Dark (1941) by Kurt Weill, Ira Gershwin, and Moss Hart, Allegro (1947) by Rodgers and Hammerstein, and Love Life (1948) by Weill and Alan Jay Lerner.
It is a term first coined by Martin Gottfried in the New Yorker magazine, to described a show which is 'theatrical and pictorial' rather than narratively intellectual.
Concept musicals began to emerge as a significant form in the early 1960s, because their fragmented approach to storytelling helped revitalize a musical theatre that was becoming formulaic and, in some people's view, stale. Both The Fantasticks and Stop the World--I Want to Get Off were influential concept musicals in 1961. It is commonly thought that the 1966 Broadway hit Cabaret, written by John Kander and Fred Ebb and directed by Harold Prince, is a 'concept musical' because of its structure, where the songs comment on the action within the narrative frame of the Kit Kat Klub. Removed from the story, these songs become effectively 'commentary' in the style of Bertholt Brecht and Kurt Weill's The Threepenny Opera, but about half of the songs are traditional "book songs" like "Perfectly Marvelous" and "So What?". It can thus be said to be the intermediary between the book musical and the concept musical! The non-linear narrative structure of Hair, which opened Off-Broadway in 1967 and then on Broadway in 1968, makes it a full-fledged concept musical.
Other uses of the phrase have been applied to the intellectually challenging work of Stephen Sondheim. Yet as John Bush Jones has pointed out, this cannot truly be the case. As Gottfried insisted that aesthetic principles and 'theatricality' take precedence as 'the concept' of the work, then the intellectual rigour and socially aware narratives of Sondheim's musicals such as Company or Sunday in the Park with George are narrative driven shows whose manner of presentation are all at once postmodern, self-referential, and challenging.
As Gottfried defined the term, it would be more applicable to works such as Andrew Lloyd Webber's Cats or Starlight Express where the elaborate staging and concept of presentation determined many other production factors.