Not To Be Confused With
Nestorianism divides Christ into two (over-separating the natures); Eutychianism blends the natures into one. Chalcedon (451) rejected both, confessing one person in two natures.
The dispute crystallized over Theotokos (“God-bearer”): if Mary bore the one person who is God the Son, the title holds. Nestorianism's over-division threatened that unity; Chalcedon answered with one hypostasis in two natures, “without confusion, without division.” (The modern Assyrian Church of the East disputes the “Nestorian” label often pinned on it.)