From BiblePortal Wikipedia

Webster's Dictionary [1]

(n.) An ancient anthem, - usually known by its Latin name tersanctus.See Tersanctus.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]

( Τρίσαγιον , Thrice Holy ) was so called because of the thrice repeating "Holy, holy, holy, Lord God of hosts," in imitation of the seraphim in the vision of Isaiah. The original of this hymn was "Holy, holy, holy, Lord God of hosts! heaven and earth are full of thy glory, who art blessed forever. Amen." Thus it is in the Constitutions, and frequently in Chrysostom. Afterwards the Church added some words to it, and sang it in this form: ῞Αγιος Θεός , Ἃγιος Ἴσχυρος , Ἃγιως Ἀθάνατος , Ἐλέησον Ἡμᾶς ("Holy God, holy Mighty, holy Immortal, have mercy upon us"). The hymn is attributed to the patriarch Proclus, in the 4th century. Theodosius the younger ordered it to be sung in the liturgy, after his vision of a child chanting it during an earthquake at Constantinople. Later still, by Anastasius the emperor, or by Peter Enapheus, bishop of Antioch, the following words were added: Σταυρωθεὶς Δἰ Ἡμᾶς , ("that was crucified for us"). This was done to oppose the heresy of the Theopaschites (q.v.), which was, in effect, to say that the whole Trinity suffered, because this hymn was commonly applied to the whole Trinity. To avoid this inconvenience, Calandio, bishop of Antioch, in the time of Zeno the emperor, made another addition to it of the words "Christ our King" reading it thus: "Holy God, holy Mighty, holy Immortal, Christ our King, that wast crucified for us, have mercy on us." These additions occasioned much confusion in the Eastern Church, while the Constantinopolitans and Western Church stiffly rejected them. It was chiefly sung in the middle of the communion service, though sometimes it was used on other occasions. After the preface this hymn was always sung, and, according to Cyril of Jerusalem, Chrysostom, and the second Council of Vaison, also at all masse mamain lenten, or of the dead. Bingham, Christ. Antiq. bk. 14 ch. 2, § 3.