The Collaborative International Dictionary of English v.0.48:

Mahdism \Mah"dism\, n.
   Belief in the coming of the Mahdi; fanatical devotion to the
   cause of the Mahdi or a pretender to that title. --
   Mah"dist, n.

         Mahdism has proved the most shameful and terrible
         instrument of bloodshed and oppression which the modern
         world has ever witnessed.                --E. N.
                                                  Bennett.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Mahdism
    n 1: belief in the appearance of the Mahdi; devotion to a Mahdi