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]