A descant, “discant,” or “discantus” is any of several different things in music, depending on the period in question. Etymologically, the word means a voice (cantus) above or removed from others. Descants originated as a form of medieval (5th - 14th centuries) music in which one singer sang a fixed melody, and others accompanied with improvisations. Those creations were sung by trained choirs, not by the congregations.
Eventually, by the Renaissance (14th - 17th centuries), descant referred generally to counterpoint. Nowadays the counterpoint meaning is the most common. The form of a descant in a congregational “hymn tune” today is commonly sung as a soprano countermelody above the melody. Its popularity emerged and grew rapidly following introduction of the 1906 English Hymnal, which some consider Ralph Vaughan Williams' magnum opus.
Although the English Hymnal of 1906 did not include descants, the influential tunes of Ralph Vaughan Williams served as a source for the earliest known “hymn tune” descants that were subsequently published. Among subsequent English composers of descants (1915 to 1934) were Ralph Vaughan Williams, Geoffrey Shaw (who worked with Vaughan Williams on the Songs of Praise hymnal), and Alan Gray.
Several of their descants appear in what is possibly the earliest hymnal to include descants, Songs of Praise (London: Oxford University Press, 1925, enlarged, 1931, reprinted 1971). During the last quarter of the 20th century, new editions of various hymnals increased the number that included descants. For example, the influential Hymnal 1940 (Episcopal) contains no descants, whereas its successor, The Hymnal 1982, contains 32.
Athelstan Riley, who was the chairman of the editorial board for the English Hymnal, subsequently compiled a collection of descants, and wrote of them –
The effect is thrilling; it gives the curious impression of an ethereal choir joining in the worship below; and those who hear it for the first time often turn and look up at the roof!"(*)