Abbas (äbäs´, ăbäs´, ăb´əs), d. 653, uncle of Muhammad the Prophet and of Ali the caliph. A wealthy merchant of Mecca, he was at first opposed to the religious movement initiated by his nephew Muhammad. In 629 he became a convert, however, and from then on he was a companion of Muhammad and the chief financial support of Islam. His descendants founded the Abbasid dynasty. The son of Abbas, Ibn Abbas (Abd Allah), was a celebrated authority on Islamic traditions and law.