The Council of Chalcedon in 451 was called by the emperor Marcian, with the empress Pulcheria behind it, to settle how the divine and the human are joined in Christ. Two answers had been condemned already and the council had to exclude both without falling into either. Nestorius had been read as dividing Christ into two subjects. Eutyches, an archimandrite of Constantinople, taught that after the union there was one nature, the human absorbed into the divine.
The definition answers with a formula of great precision. Christ is confessed in two natures, divine and human, made known without confusion, without change, without division, without separation. The first two adverbs exclude Eutyches, refusing any mixing or alteration of the natures. The second two exclude the division ascribed to Nestorius. The natures are not fused and they are not separable, and both are affirmed in one person and one hypostasis.
The council did not invent this from nothing. It quoted the creeds of Nicaea and Constantinople in full, endorsed the letters of Cyril of Alexandria, and read the Tome of Leo, the letter of the Roman bishop that supplied much of the Western phrasing. The definition presents itself as clarification of an existing faith rather than as new doctrine, which is the standard conciliar posture and is also, in this case, a claim that a large part of the church refused to accept.