Theotokos is a Greek word that means "God-bearer" or "Mother of God". It is a title assigned by the early Christian Church to the Virgin Mary at the Third Ecumenical Council. The theological significance at the time was to emphasize that Mary's son, Jesus Christ, was fully God, as well as fully human, and that Jesus' two natures (divine and human) were united in a single person of the Trinity. The competing view at that council was that Mary should be called Christotokos instead, meaning "Mother of Christ". This was the view advocated by Nestorius, then Patriarch of Constantinople. The intent behind calling her Christotokos was to restrict her role to be only the mother of "Christ's humanity" and not His Divine nature.
Calling Mary the "Mother of God" was never meant to suggest that Mary was coeternal with God, or that she existed before Jesus Christ or God existed. The Church acknowledges the mystery in the words of this ancient hymn: "He whom the entire universe could not contain was contained within your womb, O Theotokos."
The title "Theotokos" continues to be used frequently in the hymns of the Eastern Orthodox Church.