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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WordNet (r) 3.0 (2006):

defense
    n 1: (military) military action or resources protecting a
         country against potential enemies; "they died in the
         defense of Stalingrad"; "they were developed for the
         defense program" [syn: defense, defence, defensive
         measure]
    2: protection from harm; "sanitation is the best defense against
       disease" [syn: defense, defence]
    3: (sports) the team that is trying to prevent the other team
       from scoring; "his teams are always good on defense" [syn:
       defense, defence, defending team] [ant: offence,
       offense]
    4: the justification for some act or belief; "he offered a
       persuasive defense of the theory" [syn: defense, defence,
       vindication]
    5: (psychiatry) an unconscious process that tries to reduce the
       anxiety associated with instinctive desires [syn: defense
       mechanism, defense reaction, defence mechanism, defence
       reaction, defense, defence]
    6: the federal department responsible for safeguarding national
       security of the United States; created in 1947 [syn:
       Department of Defense, Defense Department, United States
       Department of Defense, Defense, DoD]
    7: the defendant and his legal advisors collectively; "the
       defense called for a mistrial" [syn: defense, defence,
       defense team, defense lawyers] [ant: prosecution]
    8: the speech act of answering an attack on your assertions;
       "his refutation of the charges was short and persuasive"; "in
       defense he said the other man started it" [syn: refutation,
       defense, defence]
    9: an organization of defenders that provides resistance against
       attack; "he joined the defense against invasion" [syn:
       defense, defence, defense force, defence force]
    10: a structure used to defend against attack; "the artillery
        battered down the defenses" [syn: defensive structure,
        defense, defence]
    11: a defendant's answer or plea denying the truth of the
        charges against him; "he gave evidence for the defense"
        [syn: defense, defence, denial, demurrer] [ant:
        criminal prosecution, prosecution]
    12: the act of defending someone or something against attack or
        injury; "a good boxer needs a good defense"; "defense
        against hurricanes is an urgent problem" [syn: defense,
        defence]

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