The Collaborative International Dictionary of English v.0.48:

Defense \De*fense"\, Defence \De*fence"\, n. [F. d['e]fense, OF.
   defense, fem., defens, masc., fr. L. defensa (cf. LL.
   defensum), from defendere. See Defend, and cf. Fence.]
   1. The act of defending, or the state of being defended;
      protection, as from violence or danger.
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            In cases of defense 't is best to weigh
            The enemy more mighty than he seems.  --Shak.
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   2. That which defends or protects; anything employed to
      oppose attack, ward off violence or danger, or maintain
      security; a guard; a protection.
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            War would arise in defense of the right. --Tennyson.
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            God, the widow's champion and defense. --Shak.
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   3. Protecting plea; vindication; justification.
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            Men, brethren, and fathers, hear ye my defense.
                                                  --Acts xxii.
                                                  1.
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   4. (Law) The defendant's answer or plea; an opposing or
      denial of the truth or validity of the plaintiff's or
      prosecutor's case; the method of proceeding adopted by the
      defendant to protect himself against the plaintiff's
      action.
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   5. Act or skill in making defense; defensive plan or policy;
      practice in self defense, as in fencing, boxing, etc.
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            A man of great defense.               --Spenser.
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            By how much defense is better than no skill. --Shak.
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   6. Prohibition; a prohibitory ordinance. [Obs.]
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            Severe defenses . . . against wearing any linen
            under a certain breadth.              --Sir W.
                                                  Temple.
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The Collaborative International Dictionary of English v.0.48:

Defense \De*fense"\, v. t.
   To furnish with defenses; to fortify. [Obs.] [Written also
   defence.]
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         Better manned and more strongly defensed. --Hales.
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Moby Thesaurus II by Grady Ward, 1.0:

152 Moby Thesaurus words for "defense":
   aegis, answer, apologetic, apologetics, apologia, apology,
   arch dam, argument, argumentation, armament, armor, arms, backfire,
   backstop, bamboo curtain, bank, bar, barrage, barrier,
   bear-trap dam, beaver dam, bicker, bickering, boom, breakwater,
   breastwork, brick wall, buffer, bulkhead, bulwark, casuistry,
   cofferdam, comeback, confutation, contention, contraremonstrance,
   controversy, counteraccusation, counterattack, counterblast,
   counterblow, countercharge, counterclaim, counterfire,
   counterinsurgency, countermeasure, counterreply, counterrevolution,
   counterstatement, counterstep, counterstroke, cover, dam, demurrer,
   denial, dike, disputation, dispute, ditch, earthwork, embankment,
   exception, exculpation, excuse, explanation, eye, fastness, fence,
   flyting, fort, fortress, gate, gravity dam, groin, guard, hassle,
   hubbub, hydraulic-fill dam, iron curtain, jam, jetty,
   justification, leaping weir, lee, levee, litigation, logjam,
   logomachy, milldam, moat, mole, mound, munitions, objection,
   paper war, parapet, passage of arms, plea, pleading, pleadings,
   polemic, polemics, portcullis, preservation, protection,
   protective custody, rampart, rationalization, rebuttal, rebutter,
   refuge, refutation, rejoinder, replication, reply, response,
   retort, return, rhubarb, riposte, roadblock, rock-fill dam,
   safeguard, safekeeping, safety, seawall, security, set-to, shade,
   shadow, shelter, shield, shutter dam, special demurrer,
   special pleading, statement of defense, stone wall, stronghold,
   surrebuttal, surrebutter, surrejoinder, verbal engagement, wall,
   war of words, ward, watchful eye, weaponry, weapons, weir,
   wicket dam, work, wrangling