The Collaborative International Dictionary of English v.0.48:

Defensive \De*fen"sive\, n.
   That which defends; a safeguard.
   [1913 Webster]

         Wars preventive, upon just fears, are true defensives.
                                                  --Bacon.
   [1913 Webster]

   To be on the defensive, To stand on the defensive, to be
      or stand in a state or posture of defense or resistance,
      in opposition to aggression or attack.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Defensive \De*fen"sive\, a. [Cf. F. d['e]fensif.]
   1. Serving to defend or protect; proper for defense; opposed
      to offensive; as, defensive armor.
      [1913 Webster]

            A moat defensive to a house.          --Shak.
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   2. Carried on by resisting attack or aggression; -- opposed
      to offensive; as, defensive war.
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   3. In a state or posture of defense. --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

defensive
    adj 1: intended or appropriate for defending against or
           deterring aggression or attack; "defensive weapons"; "a
           defensive stance" [ant: offensive]
    2: attempting to justify or defend in speech or writing [syn:
       defensive, justificative, justificatory]
    n 1: an attitude of defensiveness (especially in the phrase `on
         the defensive') [syn: defensive, defensive attitude]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "defensive":
   covering, custodial, defending, fostering, guardian, guarding,
   immunizing, parental, preventive, prophylactic, protecting,
   protective, safeguarding, screening, self-defensive,
   self-preservative, self-protective, sheltering, shielding,
   tutelary, vigilant, watchful