Show tune

A show tune is a song originally written as part of the score of a work of musical theatre, especially if the piece in question has become a standard, more or less detached in most people's minds from the original context.
Though show tunes vary in style, they do tend to share common characteristics—they usually fit the context of a story being told in the original musical, they are useful in enhancing and heightening choice moments. A particularly common form of show tune is the "I Want" song, which composer Stephen Schwartz noted as being particularly likely to have a lifespan outside the show that spawned it.
Show tunes were a major venue for popular music before the rock and roll and television era; most of the hits of such songwriters as Jerome Kern, Cole Porter, and George Gershwin came from their shows. (Even into the television and rock era, a few stage musicals managed to turn their show tunes into major pop music hits, sometimes aided by film adaptations and exposure through variety shows.) Although show tunes no longer have such a major role in popular music as they did in their heyday, they remain somewhat popular, especially among niche audiences. Show tunes make up a disproportionate part of the songs in most variations of the Great American Songbook.
The reverse phenomenon, when already popular songs are used to form the basis of a stage musical, is known as a jukebox musical.
Examples.
Particular musicals that have yielded popular “show tunes” include: