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]