Theotokos is the name the Orthodox Church gives the Virgin Mary above every other. It means God-bearer, the one who gave birth to God. The title was in use in the Church's prayer long before any council defined it.
In the fifth century Nestorius, Patriarch of Constantinople, taught that Mary should be called Christotokos, mother of Christ, but not mother of God, as though her Son's humanity and divinity could be divided. The Third Ecumenical Council, meeting at Ephesus in 431, rejected this and upheld the title Theotokos. The question was about Christ: the one born of Mary is one Person, truly God and truly man.
So the title guards the faith in the Incarnation, and the Church honours her with it in every service. Her life is kept in the feasts of the Church year: her Nativity, her Entry into the Temple, the Annunciation, and her Dormition. The Church also calls her Ever-Virgin, and sees in her the first and fullest example of a human life given wholly to God.

