What is a song exactly?

[Bob]

I thought a song was AAB form.   A least vocal, although it could imitated by an instrument.  Lyrical since it's vocally based.

There's nothing specific about form that makes it a song?  *Bob wonders if his theory or history prof was wrong...*    Maybe if it's a song, it has sections?  Phrases building up into some kind of a section so it could be labelled with a letter?


https://en.wikipedia.org/wiki/Song
Although that's Wikipedia.  Their sources aren't bad.