Incarnation is the act of clothing with flesh, or the state of being so clothed; the act of taking, or being manifested in, a human body and nature.
In the early Christian era many divisions broke out concerning the true nature of Christ. Christians believed that He was the Son of God. But how was He both Son of God and truly man?
The final definitions of the incarnation and the nature of Jesus were made by the early church at the Council of Ephesus and the Council of Chalcedon. These councils declared that Jesus was both fully God, begotten from the Father; and fully man, taking His flesh and human nature from the Virgin Mary. These two natures, human and divine, were hypostatically united into the one personhood of Jesus Christ.
The significance of the Incarnation has been extensively written-upon throughout Christian history, and is the subject of countless hymns and prayers. For instance, the Liturgy of St. John Chrysostom, as used by Orthodox and Byzantine Catholics, includes the "Hymn to the Only Begotten Son":
O only begotten Son and Word of God, Who, being immortal, deigned for our salvation to become incarnate of the holy Theotokos and ever-virgin Mary, and became man without change; You were also crucified, O Christ our God, and by death have trampled Death, being One of the Holy Trinity, glorified with the Father and the Holy Spirit— Save us!
The Athanasian Creed contains what may be considered a comprehensive definition of the Incarnation.