A show tune is a song originally written as part of the score of a work of musical theatre or musical film, especially if the piece in question has become a standard, more or less detached in most people's minds from the original context.
The Black Crook (1866), considered by some historians to be the first musical
The Great American Songbook is the loosely defined canon of significant 20th-century American jazz standards, popular songs, and show tunes.
Richard Rodgers and Lorenz Hart