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

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "mahatma":
   ace, adept, angel, anthroposophist, authority, cabalist,
   crackerjack, doctor, elder, elder statesman, esoteric, first-rater,
   genius, good hand, great, great soul, guru, holy man, illuminate,
   intellect, intellectual, lover of wisdom, magician, man of genius,
   man of intellect, man of wisdom, mandarin, master, master hand,
   mastermind, mentor, mystagogue, mystic, oracle, past master,
   philosopher, practiced hand, prodigy, rabbi, rishi, sage, saint,
   sapient, savant, scholar, seer, skilled hand, star, starets,
   supernaturalist, superstar, theosophist, thinker, topnotcher,
   transcendentalist, virtuoso, whiz, wise man, wise old man, wizard,
   yogi, yogin, yogist