The incarnation — Latin in-carno, “into flesh” — holds that in Jesus, God took on human nature fully. Not a semblance of humanity, not a God pretending to be human, but (in the classical formulation) fully God and fully human, without confusion.
The doctrine distinguishes Christianity from its Abrahamic siblings. Judaism and Islam hold strongly that God is one and other than creation; incarnation crosses a line both traditions refuse to cross. The Christian claim is that this crossing is itself the core of the good news.
For the mystics, the incarnation is not only a historical event but an ongoing possibility. Meister Eckhart: what good is it to me if Mary gave birth to the Son of God 1,400 years ago, if I do not also give birth to the Son of God in my time and in my culture?
See also Theosis — the Orthodox reading of what the incarnation makes possible.