The Collaborative International Dictionary of English v.0.48:

Heroism \Her"o*ism\ (?; 277), n. [F. h['e]ro["i]sme.]
   The qualities characteristic of a hero, as courage, bravery,
   fortitude, unselfishness, etc.; the display of such
   qualities.
   [1913 Webster]

         Heroism is the self-devotion of genius manifesting
         itself in action.                        --Hare.

   Syn: Heroism, Courage, Fortitude, Bravery, Valor,
        Intrepidity, Gallantry.

   Usage: Courage is generic, denoting fearlessness or defiance
          of danger; fortitude is passive courage, the habit of
          bearing up nobly under trials, danger, and sufferings;
          bravery is courage displayed in daring acts; valor is
          courage in battle or other conflicts with living
          opponents; intrepidity is firm courage, which shrinks
          not amid the most appalling dangers; gallantry is
          adventurous courage, dashing into the thickest of the
          fight. Heroism may call into exercise all these
          modifications of courage. It is a contempt of danger,
          not from ignorance or inconsiderate levity, but from a
          noble devotion to some great cause, and a just
          confidence of being able to meet danger in the spirit
          of such a cause. Cf. Courage.
          [1913 Webster]

WordNet (r) 3.0 (2006):

heroism
    n 1: the qualities of a hero or heroine; exceptional or heroic
         courage when facing danger (especially in battle); "he
         showed great heroism in battle"; "he received a medal for
         valor" [syn: heroism, gallantry, valor, valour,
         valorousness, valiance, valiancy]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "heroism":
   Dutch courage, bigheartedness, bigness, boldness, braveness,
   bravery, chivalrousness, chivalry, consequence,
   conspicuous gallantry, courage, courageousness, distinction,
   doughtiness, elevation, eminence, errantry, exaltation, fame,
   fearlessness, gallantness, gallantry, gallantry under fire,
   generosity, generousness, glory, great heart, greatheartedness,
   greatness, greatness of heart, heroicalness, high-mindedness,
   idealism, intrepidity, intrepidness, knight-errantry, knightliness,
   largeheartedness, liberality, liberalness, lionheartedness,
   loftiness, magnanimity, magnanimousness, majesty, manfulness,
   manhood, manliness, martial spirit, military spirit, nobility,
   noble-mindedness, nobleness, notability, openhandedness, pot-valor,
   princeliness, prominence, prowess, renown, soldierly quality,
   spirit, stalwartness, stoutheartedness, stoutness, sublimity,
   valiance, valiancy, valor, valorousness, virtue