The first three hymns of the Divine Liturgy are referred to as the antiphons.
The older custom followed by the Slavic churches is that on regular Sundays, the first two antiphons are taken from the Psalter, Psalm 102/103 (Bless the Lord, O my soul) and Psalm 145/146 (Praise the Lord, O my soul). In the Byzantine tradition, the third antiphon typically consists of the troparion of the day interspersed with psalm verses, while in the Slavic tradition, the third antiphon comes from the Beatitudes.
Following the second antiphon, a hymn by the Emperor Justinian, Only-begotten Son, is always sung. It is a hymn of faith in the divinity of Christ and his incarnation, crucifixion, and resurrection as “one of the Holy Trinity” for the salvation of men.
An antiphon consists of one or more psalm verses (or sentences from Holy Scripture), alternating with verses which contain the fundamental thought of the psalm. The name derives from the traditional practice of their being sung by two choirs, each responding antiphonally to the other.
In modern Orthodox practice, one choir or set of chanters may sing all the parts alone, but where there are two choirs or chanters alternating such hymns, the music is said to be antiphonal. The Byzantine churches tend to perform music in an antiphonal fashion more often than the Slavic. (https://orthodoxwiki.org/Antiphon)