The Collaborative International Dictionary of English v.0.48:
Sanctus \Sanc"tus\, n. [L. sanctus, p. p. of sancire.]
1. (Eccl.) A part of the Mass, or, in Protestant churches, a
part of the communion service, of which the first words in
Latin are Sanctus, sanctus, sanctus [Holy, holy, holy]; --
called also Tersanctus.
2. (Mus.) An anthem composed for these words.
Sanctus bell, a small bell usually suspended in a bell cot
at the apex of the nave roof, over the chancel arch, in
mediaeval churches, but a hand bell is now often used; --
so called because rung at the singing of the sanctus, at
the conclusion of the ordinary of the Mass, and again at
the elevation of the host. Called also Mass bell,
sacring bell, saints' bell, sance-bell, sancte