The Council at Nicæa in AD 325 had addressed the discussions and controversies over how to understand the relationship between God the Father, Jesus the Son, and the Holy Spirit. However, it did not put an end to them. Nicæa had rejected the view of Arianism, which held that Jesus was so human that he was of a different substance and nature than God. However, soon afterward the opposite idea emerged, that Jesus was not really a human being, that his human nature was so overpowered by the divine nature that his humanity was either obliterated altogether or was submersed in the divine (a position known as monophysitism, "one nature").
The Council at Chalcedon in Asia Minor (what is now Turkey) met in 451 to address the idea that Jesus lacked a human nature (along with other ecclesiastical issues). Chalcedon attempted to define a middle way that balanced Jesus’ divine and human aspects by emphasizing that Jesus had two natures unified in one person, so that he was genuinely human and yet truly divine. Chalcedon was also careful to avoid saying that Jesus was two persons, a position called Nestorianism that had already been rejected at the third ecumenical Council at Ephesus in 431.
Following, then, the holy fathers, we unite in teaching all men to confess the one and only Son, our Lord Jesus Christ. This selfsame one is perfect both in deity and in humanness; this selfsame one is also actually God and actually man, with a rational soul and a body. He is of the same reality as God as far as his deity is concerned and of the same reality as we ourselves as far as his humanness is concerned; thus like us in all respects, sin only excepted. Before time began he was begotten of the Father, in respect of his deity, and now in these "last days," for us and behalf of our salvation, this selfsame one was born of Mary the virgin, who is God-bearer in respect of his humanness.
We also teach that we apprehend this one and only Christ-Son, Lord, only-begotten - in two natures; and we do this without confusing the two natures, without transmuting one nature into the other, without dividing them into two separate categories, without contrasting them according to area or function. The distinctiveness of each nature is not nullified by the union. Instead, the "properties" of each nature are conserved and both natures concur in one "person" and in one reality. They are not divided or cut into two persons, but are together the one and only and only-begotten Word of God, the Lord Jesus Christ. Thus have the prophets of old testified; thus the Lord Jesus Christ himself taught us; thus the Creed of the Fathers* has handed down to us.
*a reference to the Nicene Creed
Another version of the Definition of Chalcedon (in more traditional language)
We, then, following the holy Fathers, all with one consent, teach men to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a rational soul and body; consubstantial with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood;
One and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten, God the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed* of the holy Fathers has handed down to us.