The Collaborative International Dictionary of English v.0.48:

contrarian \con*tra"ri*an\, a. [LL. contrarians, p. pr. of
   contrariare to oppose, fr. L. contrarius: cf. F. contrariant,
   p. pr. of contrarier to contradict. See Contrary.]
   taking an opposing view, especially a view opposite to that
   taken by the majority.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

contrarian \con*tra"ri*an\, n. [see contrarian, a..]
   a person who habitually takes a view opposite to that held by
   the majority; as, the contrarians in the stock market prefer
   to sell when most analysts advise us to buy.
   [PJC]

WordNet (r) 3.0 (2006):

contrarian
    n 1: an investor who deliberately decides to go against the
         prevailing wisdom of other investors