basilius) (1403-1472. Greek theologian. Born in Trebizond and adopted by Metropolitan Dositheus, he studied rhetoric, philosophy, and asceticism at Constantinople. He became a Basilian monk, taking the name Bessarion in 1423, was ordained deacon, and after study with the Neoplatonist George Gemistos Plethos was made abbot of St. Basil's monastery. In preparation for the Council of Ferrara/Florence, he was created archbishop of Nicea (1437) by Emperor John VII Palaeologus, whom he accompanied to the council. He labored for the union of Greek and Latin churches, which gained him unpopularity at Constantinople. Pope made him cardinal in 1439, and he eventually resided in Italy. He fulfilled important ecclesiastical missions, and in 1463 received the title of Latin patriarch of Constantinople. He presented his library to the senate of Venice, which formed the nucleus of St. Mark's Library. An enthusiastic scholar and patron of learning, he played a crucial part in the development of the Italian Renaissance.