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.
      [1913 Webster]

   2. (Mus.) An anthem composed for these words.
      [1913 Webster]

   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
      bell.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tersanctus \Ter*sanc"tus\, n. [L. ter thrice + sanctus holy.]
   (Eccl.)
   An ancient ascription of praise (containing the word "Holy"
   -- in its Latin form, "Sanctus" -- thrice repeated), used in
   the Mass of the Roman Catholic Church and before the prayer
   of consecration in the communion service of the Church of
   England and the Protestant Episcopal Church. Cf. Trisagion.
   [1913 Webster]