(disappeared 878), 12th and last imam, venerated by the Ithnā ʿAsharīyah, or Twelver sect, the main body of Shīʿite Muslims. It is believed that Muḥammad al-Mahdī al-Ḥujjah has been concealed by God (a doctrine known as ghaybah, or occultation) and that he will reappear in time as the mahdi, or messianic deliverer. See also Ithnā ʿAsharīyah.