Theotokos (/ˌθiəˈtɒkəs/; Greek: Θεοτόκος, transliterated (Greek) Theotokos, translation (Syriac-Aramaic) transliterated (Syriac): Yoldath Alloho; Latin: Mater Dei) is the Greek title of Mary, the mother of Jesus used especially in the Eastern Orthodox, Oriental Orthodox, and Eastern Catholic Churches. Its literal English translations include “God-bearer”, “Birth-Giver of God” and “the one who gives birth to God.” Less literal translations include “Mother of God.”
The ancient use of this term is emphasised in Churches of the Syriac Tradition, which have been using this title in their ancient liturgies for centuries. The Anaphora of Mari and Addai (3rd Century) and the Liturgy of St James the Just (60 AD).
Roman Catholics and Anglicans use the title “Mother of God” more often than “Theotokos.” The Council of Ephesus decreed in 431 that Mary is Theotokos because her son Jesus is both God and man: one Divine Person with two natures (Divine and human) intimately, hypostatically united.
Reference: Wikipedia, http://en.wikipedia.org/wiki/Theotоkos