The Collaborative International Dictionary of English v.0.48:

Indispensable \In`dis*pen"sa*ble\, a. [Pref. in- not +
   dispensable: cf. F. indispensable.]
   [1913 Webster]
   1. Not dispensable; impossible to be omitted, remitted, or
      spared; absolutely necessary or requisite.
      [1913 Webster]

   2. (Eccl.) Not admitting dispensation; not subject to release
      or exemption. [R.]
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            The law was moral and indispensable.  --Bp. Burnet.
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   3. Unavoidable; inevitable. [Obs.] --Fuller.
      [1913 Webster]

WordNet (r) 3.0 (2006):

indispensable
    adj 1: not to be dispensed with; essential; "foods indispensable
           to good nutrition" [ant: dispensable]
    2: absolutely necessary; vitally necessary; "essential tools and
       materials"; "funds essential to the completion of the
       project"; "an indispensable worker" [syn: essential,
       indispensable]
    3: unavoidable; "the routine but indispensable ceremonies of
       state"