Classical pieces aren't called 'songs' either. I'm not saying music without lyrics isn't real music, I'm saying that music without lyrics isn't called a 'song'. Maybe I'm wrong..
EDIT: Just looked on dictionary.com and searched for 'song'..
1. A brief composition written or adapted for singing.
2. The act or art of singing: broke into song.
So the correct term for lyricless music would be a 'tune'. Actually, I'm not to sure on that, it could be something else, but it definatly aint 'song'
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EDIT: Searched for tune, it appears it is a 'melody'. Ah well, who cares, tune, piece, composition, whatever (but not song
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AKA teh great Pet Rat.
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