The Collaborative International Dictionary of English v.0.48:

Altruist \Al"tru*ist\, n.
   One imbued with altruism; -- opposed to egoist.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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