descant

A melody sung or played above another well-known one, such as a hymn tune. The term (often as discant) is also used for a type of mediaeval polyphony, and to distinguish the highest-pitched member of a family of instruments (eg the descant recorder).

Descant or discant can refer to several different things in music, depending on the period in question;

A discant (occasionally, particularly later, written descant) is a form of medieval music in which one singer sang a fixed melody, and others accompanied with improvisations. The word in this sense comes from the term discantus supra librum (descant "by the book"), and is a form of Gregorian chant in which only the melody is notated but an improvised polyphony is understood.

Later on, the term came to mean, the treble or soprano singer in any group of voices, or the higher pitched line in a song, and eventually, by the Renaissance, referred generally to counterpoint.

Descant can also refer to the highest pitched of a group of instruments, particularly the descant viol or recorder.