The Probert Encyclopaedia of Music

DESCANT

Originally, a descant was a double song; a melody or counterpoint sung above the plain song of the tenor; a variation of an air; a variation by ornament of the main subject or plain song. The term is also applied to the upper voice in a part of music and also to the canto, cantus, or soprano voice. The term has also been used synonymously with counterpoint, or polyphony, which developed out of the French dechant, of the 12th century.
Research Descant