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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WordNet (r) 3.0 (2006):

necessity
    n 1: the condition of being essential or indispensable
    2: anything indispensable; "food and shelter are necessities of
       life"; "the essentials of the good life"; "allow farmers to
       buy their requirements under favorable conditions"; "a place
       where the requisites of water fuel and fodder can be
       obtained" [syn: necessity, essential, requirement,
       requisite, necessary] [ant: inessential,
       nonessential]

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



Bouvier's Law Dictionary, Revised 6th Ed (1856):

NECESSITY. In general, whatever makes the contrary of a thing impossible,
whatever may be the cause of such impossibilities,
     2. Whatever is done through necessity, is done without any intention,
and as the act is done without will, (q.v.) and is compulsory, the agent is
not legally responsible. Bac. Max. Reg. 5. Hence the maxim, necessity has no
law; indeed necessity is itself a law which cannot be avoided nor infringed.
Clef des Lois Rom. h.t.; Dig 10, 3, 10, 1; Com. Dig. Pleader, 3 M 20, 3 M
30.
     3. It follows, then, that the acts of a man in violation of law., or to
the injury of another, may be justified by necessity, because the actor has
no will to do or not to do the thing, he is a mere tool; but, it is
conceived, this necessity must be absolute and irresistible, in fact, or so
presumed in point of law.
     4. The cases which are justified by necessity, may be classed as
follows:
     I. For the preservation of life; as if two persons are on the same
plank, and one must perish, the survivor is justified in having thrown off
the other, who was thereby drowned. Bac. Max, Reg. 5.
     5.-2. Obedience by a person subject to the power of another; for
example, if a wife should commit a larceny with her husband, in this case
the law presumes she acted by coercion of her husband, and, being compelled,
by necessity, she is justifiable. 1 Russ. Cr. 16, 20; Bac. Max. Reg. 5.
     6.-3. Those cases which arise from the act of God, or inevitable
accident, or from the act of man, as public enemies. Vide Act of God;
Inevitable Accident and also 15 Vin. Ab. 534 Dane's Ab h.t.; 2 Stark. Ev.
713; Marsh. Ins. b. 1, c. 6, s. 3 Jacob's Intr. to. Com. Law. Reg. 74.
     7.-4. There is another species of necessity. The actor in these cases
is not compelled to do the act whether he will or not, but he has no choice
left but to do the act which may be injurious to another, or to lose the
total use of his property. For example, when a man's lands are surrounded by
those of others, so that he cannot enjoy them without trespassing on his
neighbors. The way which is thus obtained, is called a way of necessity.
Gale and Whatley on Easements, 71; 11 Co. 52; Hob. 234; 1 Saund. 323, note.
See 3 Rawle, R. 495; 3 M'Cord, R. 131; Id. 170; 14 Mass. R. 56; 2 B. & C.
96; 2 Bing. R. 76; 8 T. R. 50; Cro. Jac. 170; 2 Roll. Ab. 60; 3 Kent, Com.
423; 3 Rawle's R. 492; 1 Taunt. R. 279; 8 Taunt. R. 24; ST. R. 50; Ham. N.
P. 198; Cro. Jac. 170; 2 Bouv. Inst. n. 1637; and Way.