Source : Webster's Revised Unabridged Dictionary (1913)

Self-defense \Self`-de*fense"\, n.
   The act of defending one's own person, property, or
   reputation.

   {In self-defense} (Law), in protection of self, -- it being
      permitted in law to a party on whom a grave wrong is
      attempted to resist the wrong, even at the peril of the
      life of the assailiant. --Wharton.