Good points CarolC.
I suppose that one could argue that an opera contains a collection of songs (what is the posh word - arias?) but that still leaves bits in between that are sung that are more like dialogue that I can not imagine anyone classing as a song. Where does that leave us? What is that missing requirement?
I missed the thread that you mentioned but when is a tune not a tune? In the case of an arrangement, I would consider the tune to be the dominant melody line, the one you would sing if somebody asked how does that piece go? but I am not convinced that covers everything.