Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. (Eccl.) Not admitting dispensation; not subject to release
      or exemption. [R.]

            The law was moral and indispensable.  --Bp. Burnet.

   3. Unavoidable; inevitable. [Obs.] --Fuller.

Source : WordNet®

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}]
     3: unavoidable; "the routine but indispensable ceremonies of
        state"