1) Christian prayer 
The Trisagion (Τρισάγιον `Thrice Holy`), sometimes called by its opening line Agios O Theos, is a standard hymn of the Divine Liturgy in most of the Eastern Orthodox, Oriental Orthodox and Eastern Catholic churches. The Latin name Tersanctus or Ter Sanctus is sometimes used to refer to this hymn, although this name is also sometimes use.....
• (n.) An ancient anthem, -- usually known by its Latin name tersanctus.See Tersanctus.