The Collaborative International Dictionary of English v.0.48:

Necessary \Nec"es*sa*ry\, a. [L. necessarius, from necesse
   unavoidable, necessary; of uncertain origin: cf. F.
   n['e]cessaire.]
   1. Such as must be; impossible to be otherwise; not to be
      avoided; inevitable.
      [1913 Webster]

            Death, a necessary end,
            Will come when it will come.          --Shak.
      [1913 Webster]

   2. Impossible to be otherwise, or to be dispensed with,
      without preventing the attainment of a desired result;
      indispensable; requisite; essential. "'T is necessary he
      should die." --Shak.
      [1913 Webster]

            A certain kind of temper is necessary to the
            pleasure and quiet of our minds.      --Tillotson.
      [1913 Webster]

   3. Acting from necessity or compulsion; involuntary; --
      opposed to free; as, whether man is a necessary or a
      free agent is a question much discussed.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Necessary \Nec"es*sa*ry\, n.; pl. Necessaries.
   1. A thing that is necessary or indispensable to some
      purpose; something that one can not do without; a
      requisite; an essential; -- used chiefly in the plural;
      as, the necessaries of life.
      [1913 Webster]

   2. A privy; a water-closet.
      [1913 Webster]

   3. pl. (Law) Such things, in respect to infants, lunatics,
      and married women, as are requisite for support suitable
      to station.
      [1913 Webster]

WordNet (r) 3.0 (2006):

necessary
    adj 1: absolutely essential [ant: unnecessary, unneeded]
    2: unavoidably determined by prior circumstances; "the necessary
       consequences of one's actions"
    n 1: 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:

94 Moby Thesaurus words for "necessary":
   absolute, apodictic, backhouse, binding, bound, called for,
   cardinal, certain, choiceless, clear, clear and distinct,
   clear as day, closet, compelling, compulsory, conclusive,
   constraining, crapper, de rigueur, decisive, definite, determinate,
   dictated, essential, exigent, fated, fateful, fundamental,
   imperative, imperious, important, importunate, indefeasible,
   indicated, indispensable, ineluctable, ineludible, inerrable,
   inerrant, inescapable, inevasible, inevitable, inexorable,
   infallible, inflexible, involuntary, irreducible, irreductible,
   irreplaceable, irresistible, irrevocable, jakes, mandatory,
   momentous, must, necessitous, needed, needful, obligatory, office,
   outhouse, peremptory, perfectly sure, positive, predestined,
   predetermined, prerequisite, relentless, required, requisite,
   resistless, significant, sure, sure as death, sure as fate,
   sure-enough, true, unambiguous, unavoidable, uncontrollable,
   undeflectable, unequivocal, unerring, unevadable, unforgoable,
   univocal, unmistakable, unpreventable, unstoppable, unyielding,
   urgent, vital, wanted, without choice