Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. The condition of being needy or necessitous; pressing
      need; indigence; want.

            Urge the necessity and state of times. --Shak.

            The extreme poverty and necessity his majesty was
            in.                                   --Clarendon.

   3. That which is necessary; a necessary; a requisite;
      something indispensable; -- often in the plural.

            These should be hours for necessities, Not for
            delights.                             --Shak.

            What was once to me Mere matter of the fancy, now
            has grown The vast necessity of heart and life.
                                                  --Tennyson.

   4. That which makes an act or an event unavoidable;
      irresistible force; overruling power; compulsion, physical
      or moral; fate; fatality.

            So spake the fiend, and with necessity, The tyrant's
            plea, excused his devilish deeds.     --Milton.

   5. (Metaph.) The negation of freedom in voluntary action; the
      subjection of all phenomena, whether material or
      spiritual, to inevitable causation; necessitarianism.

   {Of necessity}, by necessary consequence; by compulsion, or
      irresistible power; perforce.

   Syn: See {Need}.

Source : WordNet®

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: {essential}, {requirement}, {requisite}, {necessary}]
        [ant: {inessential}]