The fourth ecumenical council, convened in AD 451 at Chalcedon (across the Bosporus from Constantinople). About 520 bishops attended — the largest council of the patristic era. Its task was to settle the question of Christ's natures in the wake of Nestorianism (which over-divided the two natures) and Eutychianism / monophysitism (which collapsed them into one). The Chalcedonian Definition confesses that Christ is "one Person in two natures, without confusion, without change, without division, without separation" — the famous four negatives that fence the mystery.
Chalcedon gave the Church its settled Christology. The formula safeguards four non-negotiables: (1) Jesus is fully God; (2) Jesus is fully man; (3) Jesus is one Person, not two; (4) the two natures are neither mixed (His humanity isn't absorbed into deity) nor separated (we don't have two Christs). Every Christological heresy violates at least one of these. Arianism denies #1. Docetism and Apollinarianism deny #2. Nestorianism denies #3. Eutychianism denies #4. Modern cults usually pick their favorite error. Chalcedon doesn't explain how the hypostatic union works — it confesses the mystery while marking the limits past which thought dare not go. The Oriental Orthodox (Copts, Armenians, Ethiopians) rejected Chalcedon over the word "in two natures," preferring "from two natures"; contemporary dialogue suggests the difference may have been more linguistic than substantive. For catholic Christianity, Chalcedon stands as the guardrail on the cliff of Christology.