Trisagion is my word for today. As you may guess, the "tri" in Trisagion means three. It is a phrase describing God that is used three times in our Eucharistic Prayer, the Sanctus: Holy, Holy, Holy, Lord God Almighty...
It has been used since the early centuries. Sometimes it is called by the Greek, Agios O Theos. This is a standard hymn at Eastern Orthodox's Masses.
