The Collaborative International Dictionary of English v.0.48:

Necessitate \Ne*ces"si*tate\, v. t. [imp. & p. p.
   Necessitated; p. pr. & vb. n. Necessitating.] [Cf. L.
   necessitatus, p. p. of necessitare, and F. n['e]cessiter. See
   Necessity.]
   1. To make necessary or indispensable; to render unavoidable.
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            Sickness [might] necessitate his removal from the
            court.                                --South.
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            This fact necessitates a second line. --J. Peile.
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   2. To reduce to the necessity of; to force; to compel.
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            The Marquis of Newcastle, being pressed on both
            sides, was necessitated to draw all his army into
            York.                                 --Clarendon.
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WordNet (r) 3.0 (2006):

necessitate
    v 1: require as useful, just, or proper; "It takes nerve to do
         what she did"; "success usually requires hard work"; "This
         job asks a lot of patience and skill"; "This position
         demands a lot of personal sacrifice"; "This dinner calls
         for a spectacular dessert"; "This intervention does not
         postulate a patient's consent" [syn: necessitate, ask,
         postulate, need, require, take, involve, call
         for, demand] [ant: eliminate, obviate, rid of]
    2: cause to be a concomitant