Leo's Tome was a document sent by Pope Leo I to Flavian of Constantinople, explaining the position of the Papacy in matters of Christology. The text confesses that Christ has two natures, both fully human and fully divine. The letter was a topic of debate at the Council of Chalcedon in 451 being eventually accepted as a doctrinal explanation of the nature of the Person of Christ. The letter was written in response to Flavian, Patriarch of Constantinople, who had excommunicated Eutyches, who also wrote to the Pope to appeal the excommunication. Source: Wikipedia (en)