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.]
      [1913 Webster]

            The law was moral and indispensable.  --Bp. Burnet.
      [1913 Webster]

   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"

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "indispensable":
   bare necessities, basic, bedrock, call for, called for, cardinal,
   central, compelling, compulsory, crucial, demand, demand for,
   demanded, desideration, desideratum, essential, essentials, focal,
   fundamental, imperative, important, in demand, indicated,
   ineluctable, inescapable, irreducible, irreductible, irreplaceable,
   key, life-and-death, life-or-death, mandatory, material, must,
   must item, necessaries, necessary, necessities, necessitous,
   necessity, need, need for, needed, needful, obligatory, occasion,
   of the essence, of vital importance, prerequirement, prerequisite,
   required, requirement, requisite, requisition, sine qua non,
   substantive, the necessary, the needful, unavoidable, unforgoable,
   urgent, vital, want, wanted