The Collaborative International Dictionary of English v.0.48:

Defender \De*fend"er\ (d[-e]*f[e^]nd"[~e]r), n. [Cf. Fender.]
   One who defends; one who maintains, supports, protects, or
   vindicates; a champion; an advocate; a vindicator.
   [1913 Webster]

         Provinces . . . left without their ancient and puissant
         defenders.                               --Motley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

defender
    n 1: a person who cares for persons or property [syn:
         defender, guardian, protector, shielder]
    2: a fighter who holds out against attack [syn: defender,
       withstander]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "defender":
   Maecenas, abettor, admirer, advocate, aficionado, angel, apologete,
   apologist, apologizer, backer, buff, champion, dependence,
   encourager, endorser, exponent, fan, favorer, friend at court,
   guard, justifier, lover, mainstay, maintainer, paladin, paranymph,
   partisan, patron, patroness, pillar, pleader, promoter, proponent,
   protagonist, protector, protectress, reliance, rock, safekeeper,
   second, seconder, sectary, sider, sponsor, stalwart, standby,
   successful advocate, support, supporter, sustainer, sympathizer,
   tower, tower of strength, upholder, vindicator, votary,
   well-wisher, whitewasher



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DEFENDER, canon law. The name by which the defendant or respondent is known
in the ecclesiastical courts.