the definition of faith of the Council of Chalcedon are the following:
Following the holy fathers we teach with one voice that the Son of God and our Lord Jesus Christ is to be confessed as one and the same [Person], and He is perfect in Divinity and perfect in Humanity, true God and true Man, of a rational soul and [human] body consisting, of one essence with the Father as touching His Divinity and of one essence with us as touching His Humanity; made in all things like unto us, with the exception of sin only; begotten of His Father before all ages according to His Divinity: but in these last days, for us men and for our salvation, born [into the world] of the Virgin Mary, Theotokos, according to His Humanity. This one and the same Jesus Christ, the only-begotten Son [of God] must be confessed to be in two natures, without mixture and without change, without separation and without division [i.e., without fusing together Divinity and Humanity so that the proper characteristics of each are changed or lost; and also without separating them in such a way that there might be considered to be two Sons and not One Son only] and that without the distinction of natures being removed by such union, but rather that the peculiar property of each nature being preserved and being united in one Person and Hypostasis, not separated or divided into two persons, but one and the same Son and only begotten, God the Word, our Lord Jesus Christ, as the Prophets of old have spoken concerning Him [e.g., the Immanuel of Is 7.14], and as Jesus Christ has taught us, and as the Creed of the fathers has delivered to us.
adding from Fr Thomas Hopko in 'The Orthodox Faith'
The virtue of the fourth council, in the Orthodox view, is that it defines very clearly the fact that when the Son of God was born as a man from the Virgin Mary, Theotokos, He did not cease to be God or change in His Divinity, while becoming a complete and perfect man in His incarnate Humanity. For salvation itself requires the perfect union of Divinity and Humanity in the one Person of Jesus Christ; union where God is God and Man is Man, and yet where the two become one in perfect unity: without fusion or change, and without division or separation.