A Dictionary of Music and Musicians/Discant

DISCANT, dis-cantus, a double song; originally the melody or 'counterpoint' sung with a plain-song; thence the upper voice or leading melody in a piece of part-music; and thence the canto, cantus, or soprano voice, which was formerly—as late as Mendelssohn, who used to say he had learnt it from Zelter—written in the C clef.

Thus in earlier English the word 'discant' or 'descant' means an air:

'And sprightly voice sweet descant sing.'

And the violin, because it took the upper part in the quartet, was called the 'diskant- Violin.'