When you’re writing that cultural event report based on last night’s symphony concert, don’t call the music performed “songs.” Songs are strictly pieces of music which are sung—by singers. Instrumental numbers may be called “works,” “compositions,” or even “pieces.” Be careful, though: a single piece may have several different movements; and it would be wrong to refer to the Adagio of Beethoven’s Moonlight Sonata as a “piece.” It’s just a piece of a piece.
See also music/singing.
BUY THE BOOK!