A Mohammedan dynasty which assumed the title of caliphs and ruled N. Africa and Egypt, and later Syria and Palestine, between the 10th and 12th centuries inclusive; they derived their name from the claim (now discredited) of their founder, Obeidallah Almahdi, to be descended from Fatima, daughter of Mahomet and wife of Ali; they were finally expelled by Saladin in 1169.