The Collaborative International Dictionary of English v.0.48:

Invalidity \In`va*lid"i*ty\, n. [Cf. F. invalidit['e], LL.
   invaliditas lack of health.]
   [1913 Webster]
   1. Lack of validity or cogency; lack of legal force or
      efficacy; invalidness; as, the invalidity of an agreement
      or of a will.
      [1913 Webster]

   2. Lack of health; infirmity. [Obs.] --Sir W. Temple.
      [1913 Webster]

WordNet (r) 3.0 (2006):

invalidity
    n 1: illogicality as a consequence of having a conclusion that
         does not follow from the premisses [syn: invalidity,
         invalidness]