Trisagion
The Trisagion (Greek: Τρισάγιον, lit. 'Thrice Holy'), sometimes called by its incipit Agios O Theos (Greek: Άγιος ο Θεός) is a standard hymn of ancient origin of the Divine Liturgy in most of the Eastern Orthodox, Western Orthodox, Oriental Orthodox, and Eastern Catholic Churches. In Byzantine-rite Churches, the Trisagion is chanted immediately before the Prokeimenon and the Epistle Reading.