The Collaborative International Dictionary of English v.0.48:

Defend \De*fend"\ (d[-e]*f[e^]nd"), v. t. [imp. & p. p.
   Defended; p. pr. & vb. n. Defending.] [F. d['e]fendre, L.
   defendere; de- + fendere (only in comp.) to strike; perh.
   akin to Gr. qei`nein to strike, and E. dint. Cf. Dint,
   Defense, Fend.]
   1. To ward or fend off; to drive back or away; to repel. [A
      Latinism & Obs.]
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            Th' other strove for to defend
            The force of Vulcan with his might and main.
                                                  --Spenser.
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   2. To prohibit; to forbid. [Obs.] --Chaucer.
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            Which God defend that I should wring from him.
                                                  --Shak.

   3. To repel danger or harm from; to protect; to secure
      against attack; to maintain against force or argument; to
      uphold; to guard; as, to defend a town; to defend a cause;
      to defend character; to defend the absent; -- sometimes
      followed by from or against; as, to defend one's self
      from, or against, one's enemies.
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            The lord mayor craves aid . . . to defend the city.
                                                  --Shak.
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            God defend the right!                 --Shak.
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            A village near it was defended by the river.
                                                  --Clarendon.
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   4. (Law.) To deny the right of the plaintiff in regard to
      (the suit, or the wrong charged); to oppose or resist, as
      a claim at law; to contest, as a suit. --Burrill.

   Syn: To Defend, Protect.

   Usage: To defend is literally to ward off; to protect is to
          cover so as to secure against approaching danger. We
          defend those who are attacked; we protect those who
          are liable to injury or invasion. A fortress is
          defended by its guns, and protected by its wall.
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                As birds flying, so will the Lord of hosts
                defend Jerusalem; defending also he will deliver
                it.                               --Is. xxxi. 5.
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                Leave not the faithful side
                That gave thee being, still shades thee and
                protects.                         --Milton.
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WordNet (r) 3.0 (2006):

defend
    v 1: argue or speak in defense of; "She supported the motion to
         strike" [syn: defend, support, fend for]
    2: be on the defensive; act against an attack [ant: assail,
       attack]
    3: protect against a challenge or attack; "Hold that position
       behind the trees!"; "Hold the bridge against the enemy's
       attacks" [syn: defend, guard, hold]
    4: fight against or resist strongly; "The senator said he would
       oppose the bill"; "Don't fight it!" [syn: fight, oppose,
       fight back, fight down, defend]
    5: protect or fight for as a champion [syn: champion,
       defend]
    6: be the defense counsel for someone in a trial; "Ms. Smith
       will represent the defendant" [syn: defend, represent]
       [ant: prosecute]
    7: state or assert; "He maintained his innocence" [syn:
       maintain, defend]

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "defend":
   account, advocate, air, allege in support, answer, argue,
   argue for, arm, armor, assert, avert, back, battle, bless, bulwark,
   champion, claim, cloak, compass about, conserve, contend,
   contend for, copyright, counter, cover, cushion, ensure, espouse,
   explain, express, fence, fend, fend off, fight, fight for, flank,
   fortify, guarantee, guard, guard against, harbor, haven, insure,
   justify, keep, keep from harm, keep safe, maintain, make a plea,
   make safe, nestle, oppose, parry, patent, plead for, police,
   preserve, prevent, protect, rationalize, rebut, refute, register,
   reply, resist, respond, ride shotgun for, riposte, safeguard, save,
   say in defense, screen, secure, shelter, shield, shroud, speak for,
   speak up for, stand by, stand up for, stick up for, support,
   sustain, underwrite, uphold, urge reasons for, utter, vent,
   vindicate, voice, war, ward, ward off, warrant, watch over,
   withstand