The Collaborative International Dictionary of English v.0.48:

Inability \In`a*bil"i*ty\, n. [Pref. in- not + ability: cf. F.
   inhabilet['e]. See Able, and cf. Unable.]
   The quality or state of being unable; lack of ability; lack
   of sufficient power, strength, resources, or capacity.
   [1913 Webster]

         It is not from an inability to discover what they ought
         to do, that men err in practice.         --Blair.

   Syn: Impotence; incapacity; incompetence; weakness;
        powerlessness; incapability. See Disability.
        [1913 Webster]

WordNet (r) 3.0 (2006):

inability
    n 1: lack of ability (especially mental ability) to do something
         [ant: ability, power]
    2: lacking the power to perform [syn: inability, unfitness]
       [ant: ability]