What is "trisagion"

Trisagion \Tris*ag"i*on\, n. [NL., fr. Gr. ? thrice holy; ? thrice + ? holy.] (Eccl.) An ancient anthem, -- usually known by its Latin name tersanctus.See Tersanctus.

The Trisagion , 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 used to refer to the Sanctus; it is the latter, a different formula, which is used in Western Christianity in the Mass.

In churches which use the Byzantine Rite, the Trisagion is chanted immediately before the Prokeimenon and the Epistle reading. It is also included in a set of prayers named for it, called the Trisagion Prayers, which forms part of numerous services (the Hours, Vespers, Matins, and as part of the opening prayers for most services).

It is popular also in the Latin Church, where it is a part of the supplication, the chaplet of divine mercy and other prayers.