Wikipedia, which as we all know is never wrong about anything, defines a show tune thusly: "A show tune is a popular song originally written as part of the score of a show (or stage musical), especially if the piece in question has become a standard, more or less detached in most people's minds from the original context." I would like to suggest that's a rotten definition…or at least, an outdated one. It sure doesn't apply to a lot of what I hear on the Show Tunes music channel I hear on my TV or the Broadway channel on Sirius XM radio. To give just one example, both often play "Always Look on the Bright Side of Life," from Spamalot or "Springtime for Hitler" from The Producers. These were two big, show-stopping numbers in productions that won for Best Musical…
…but they don't fit that definition. They weren't written as part of the score of a stage musical. Both were written for movies.
The definition of a show tune on these audio channels seems to be anything that was performed on a Broadway stage. This makes the biggest hits of Frankie Valli and the Four Seasons into show tunes. They were performed in Jersey Boys. All the Leiber and Stoller hits like "Jailhouse Rock" are show tunes because they were in Smokey Joe's Cafe. A lot of disco records from the seventies are show tunes because they were in Priscilla, Queen of the Desert.
I got to thinking about this a few weeks ago as I was flipping the dial on my Sirius XM radio. On the seventies channel, I heard ABBA performing "Mamma Mia." Then I switched over to the Broadway channel and there was the Broadway cast of Mamma Mia singing "Mamma Mia." I may have been confused but I think a few weeks later on the Broadway channel, I heard "Waterloo," which is also in the stage musical of Mamma Mia, but this time they were playing the ABBA original. (The rule seems to be that it's the song that makes it a show tune, not who sings it.)
What I'm getting at is that the line of demarcation is beyond fuzzy here. I guess it was always fuzzy because, for example, several songs the Gershwins wrote for movies were later used in stage productions. But with all these "jukebox" musicals built around the hits of earlier decades, the line is blurred beyond recognition. Now, I turn on my radio and I hear someone singing "Yesterday." Is this the sixties channel I'm on? Or the Broadway channel because that song was in Beatlemania? Beats me.