The Collaborative International Dictionary of English v.0.48:

Philanthropist \Phi*lan"thro*pist\, n. [Gr. fila`nqrwpos; fi`los
   loving + 'a`nqrwpos man: cf. F. philanthrope.]
   1. One who practices philanthropy; one who loves mankind, and
      seeks to promote the good of others; especially, a wealthy
      individual who donates large amounts of money to
      charitable or philanthropic causes. Opposite of
      misanthrope.
      [1913 Webster]

WordNet (r) 3.0 (2006):

philanthropist
    n 1: someone who makes charitable donations intended to increase
         human well-being [syn: philanthropist, altruist]

The Devil's Dictionary (1881-1906):

PHILANTHROPIST, n.  A rich (and usually bald) old gentleman who has
trained himself to grin while his conscience is picking his pocket.