The Collaborative International Dictionary of English v.0.48:

Indispensability \In`dis*pen`sa*bil"i*ty\, n. [Cf. F.
   indispensabilit['e].]
   Indispensableness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indispensability
    n 1: the quality possessed by something that you cannot possibly
         do without [syn: indispensability, indispensableness,
         vitalness] [ant: dispensability, dispensableness]