Webster's Revised Unabridged Dictionary · 1913

Descant

Descant (des"kant) , noun

[Old French descant, deschant, French déchant, discant, Late Latin discantus, from Latin dis + cantus singing, melody, from canere to sing. See Chant, and compare Descant, v. i., Discant.]

1.
(a) (Music) Originally, 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.
(b)
(Music) The upper voice in part music.
(c)
(Music) The canto, cantus, or soprano voice; the treble. — Grove
Twenty doctors expound one text twenty ways, as children make descant upon plain song. — Tyndale
She [the nightingale] all night long her amorous descant sung. — Milton

The term has also been used synonymously with counterpoint, or polyphony, which developed out of the French déchant, of the 12th century.

2.
A discourse formed on its theme, like variations on a musical air; a comment or comments.
Upon that simplest of themes how magnificent a descant! — De Quincey

Descant (des*kant") , intransitive verb

[From descant; n.; or directly from Old French descanter, deschanter; Latin dis- + cantare to sing.]

1.
To sing a variation or accomplishment.
2.
To comment freely; to discourse with fullness and particularity; to discourse at large.
A virtuous man should be pleased to find people descanting on his actions. — Addison