Source : Webster's Revised Unabridged Dictionary (1913)

Indispensability \In`dis*pen`sa*bil"i*ty\, n. [Cf. F.
   indispensabilit['e].]
   Indispensableness.

Source : WordNet®

indispensability
     n : the quality possessed by something that you cannot possibly
         do without [syn: {indispensableness}, {vitalness}] [ant:
         {dispensability}, {dispensability}]