"Non-Chalcedonian" refers to Christian groups that do not accept the decisions made at the Council of Chalcedon in 451 AD. This council defined the two natures of Christ as both fully divine and fully human, a doctrine that many Non-Chalcedonian churches reject. Instead, they emphasize the unity of Christ's nature, believing that He is one divine nature. The main Non-Chalcedonian churches include the Coptic Orthodox Church, the Armenian Apostolic Church, and the Ethiopian Orthodox Tewahedo Church. These churches often have distinct theological beliefs and practices, and they celebrate their own traditions and liturgies, which differ from those of Chalcedonian Christianity.