The Collaborative International Dictionary of English v.0.48:

Altruism \Al"tru*ism\, n. [F. altruisme (a word of Comte's), It.
   altrui of or to others, fr. L. alter another.]
   Regard for others, both natural and moral; devotion to the
   interests of others; brotherly kindness; -- opposed to
   egoism or selfishness. [Recent] --J. S. Mill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

altruism
    n 1: the quality of unselfish concern for the welfare of others
         [syn: altruism, selflessness] [ant: egocentrism,
         egoism, self-centeredness, self-concern, self-
         interest]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "altruism":
   BOMFOG, Benthamism, Christian charity, Christian love, agape,
   beneficence, benevolence, benevolent disposition, benevolentness,
   bigheartedness, brotherly love, caritas, charitableness, charity,
   commitment, consecration, dedication, devotion, disinterest,
   disinterestedness, do-goodism, flower power, generosity, giving,
   goodwill, grace, greatheartedness, humaneness, humanitarianism,
   humanity, humility, largeheartedness, love, love of mankind,
   modesty, philanthropism, philanthropy, sacrifice, self-abasement,
   self-abnegation, self-denial, self-devotion, self-effacement,
   self-forgetfulness, self-immolation, self-neglect,
   self-neglectfulness, self-renouncement, self-sacrifice,
   self-subjection, selflessness, unacquisitiveness, unpossessiveness,
   unselfishness, utilitarianism, welfarism, well-disposedness