FUUUUCK!!! When will people understand that if something doesn't have words
IT'S NOT A FUCKING SONG, IT'S A PIECE!!! A song is music which has or has had words written to be sung with the music. A piece is music that has never had words written to go along with it.
Also, if you're talking about a symphony, such as Dvořák's 9th
New World Symphony, the pieces that make up the symphony are called movements, and are often simply called by the tempo of the movement. Well, that was pre-Romantic. Starting in the Romantic period composers would sometimes name their pieces by name, and not just by tempo.
Ok. I got that off my chest.