The Collaborative International Dictionary of English v.0.48:

Mahatma \Ma*hat"ma\, n. [Skr. mah[=a]tman, lit., great-souled,
   wise.] (Theosophy)
   One of a class of sages, or "adepts," reputed to have
   knowledge and powers of a higher order than those of ordinary
   men.

   Note: The title was popularly applied to Mohandas Karamchand
         Gandhi in tribute to his wisdom, though not implying
         any supernatural powers; he is thus often referred to
         as Mahatma Ghandi or The Mahatma. He was most noted for
         his advocacy of non-violence in resisting oppression,
         and played an important role in convincing Great
         Britain to grant independence to India and Pakistan. --
         Ma*hat"ma*ism, n.
         [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

mahatma
    n 1: (Hinduism) term of respect for a brahmin sage