Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. Want of health; infirmity. [Obs.] --Sir W. Temple.

Source : WordNet®

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