a member of a dynasty of caliphs ruling at Baghdad, a.d. 750–1258, governing most of the Islamic world and claiming descent from Abbas, uncle of Muhammad.
Ab·bas·sid (ə-bās'ĭd', āb'ə-sĭd') An Arabic dynasty (750-1258) that expanded the Muslim empire. It was named for al-Abbas (566?-652), paternal uncle of the prophet Muhammad.