The Collaborative International Dictionary of English v.0.48:

Mahdi \Mah"di\, n. [Ar., guide, leader.]
   Among Mohammedans, the last imam or leader of the faithful.
   The Sunni, the largest sect of the Mohammedans, believe that
   he is yet to appear.
   [1913 Webster]

   Note: The title has been taken by several persons in
         countries where Mohammedanism prevails, -- notably by
         Mohammad Ahmed, who overran the Egyptian Sudan, and in
         1885 captured Khartum, his soldiers killing General
         Gordon, an Englishman, who was then the Egyptian
         governor of the region.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Mahdi
    n 1: (Islam) a messianic leader who (according to popular Muslim
         belief) will appear before the end of the world and restore
         justice and religion