The Collaborative International Dictionary of English v.0.48:

Necessity \Ne*ces"si*ty\, n.; pl. Necessities. [OE. necessite,
   F. n['e]cessit['e], L. necessitas, fr. necesse. See
   Necessary.]
   1. The quality or state of being necessary, unavoidable, or
      absolutely requisite; inevitableness; indispensableness.
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   2. The condition of being needy or necessitous; pressing
      need; indigence; want.
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            Urge the necessity and state of times. --Shak.
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            The extreme poverty and necessity his majesty was
            in.                                   --Clarendon.
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   3. That which is necessary; a necessary; a requisite;
      something indispensable; -- often in the plural.
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            These should be hours for necessities,
            Not for delights.                     --Shak.
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            What was once to me
            Mere matter of the fancy, now has grown
            The vast necessity of heart and life. --Tennyson.
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   4. That which makes an act or an event unavoidable;
      irresistible force; overruling power; compulsion, physical
      or moral; fate; fatality.
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            So spake the fiend, and with necessity,
            The tyrant's plea, excused his devilish deeds.
                                                  --Milton.
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   5. (Metaph.) The negation of freedom in voluntary action; the
      subjection of all phenomena, whether material or
      spiritual, to inevitable causation; necessitarianism.
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   Of necessity, by necessary consequence; by compulsion, or
      irresistible power; perforce.
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   Syn: See Need.
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Moby Thesaurus II by Grady Ward, 1.0:

126 Moby Thesaurus words for "necessity":
   absolute certainty, absoluteness, act of God, assurance,
   assuredness, bare cupboard, bare necessities, bare subsistence,
   beggarliness, beggary, call, call for, cause, certain knowledge,
   certainness, certainty, certitude, coaction, coercion, compulsion,
   compulsiveness, condition, constraint, dead certainty, decree,
   definiteness, demand, demand for, deprivation, desideration,
   desideratum, destitution, determinacy, determinateness, duress,
   empty purse, enforcement, essential, essentials, exigency, fate,
   fatefulness, force majeure, forcing, foredestiny,
   foregone conclusion, foreknowledge, foreordination,
   grinding poverty, gripe, hand-to-mouth existence, homelessness,
   impoverishment, indefeasibility, indigence, indispensable,
   indispensableness, ineluctability, inerrability, inerrancy,
   inescapableness, inevasibleness, inevitability,
   inevitable accident, inevitableness, inexorability, infallibilism,
   infallibility, inflexibility, irresistibility, irrevocability,
   lack, mendicancy, moneylessness, must, must item, necessaries,
   necessities, necessitousness, need, need for, needfulness,
   neediness, nonambiguity, noncontingency, obligation, obligement,
   occasion, pauperism, pauperization, penury, pinch, positiveness,
   predestination, predetermination, preordination, prerequirement,
   prerequisite, prescience, privation, probatum, proved fact,
   relentlessness, requirement, requisite, requisiteness, requisition,
   restraint, sine qua non, sureness, surety, the necessary,
   the needful, truth, unambiguity, unavoidable casualty,
   unavoidableness, uncontrollability, undeflectability,
   unequivocalness, univocity, unmistakableness, unpreventability,
   unyieldingness, vis major, want