Source : Webster's Revised Unabridged Dictionary (1913)

Impossibility \Im*pos`si*bil"i*ty\, n.; pl. {Impossibilities}.
   [L. impossibilitas: cf. F. impossibilit['e].]
   1. The quality of being impossible; impracticability.

            They confound difficulty with impossibility.
                                                  --South.

   2. An impossible thing; that which can not be thought, done,
      or endured.

            Impossibilities! O, no, there's none. --Cowley.

   3. Inability; helplessness. [R.] --Latimer.

   {Logical impossibility}, a condition or statement involving
      contradiction or absurdity; as, that a thing can be and
      not be at the same time. See {Principle of Contradiction},
      under {Contradiction}.

Source : WordNet®

impossibility
     n 1: incapability of existing or occurring [syn: {impossibleness}]
          [ant: {possibility}]
     2: an alternative that is not available [syn: {impossible
        action}]