Source : Webster's Revised Unabridged Dictionary (1913)

Convertibility \Con*vert`i*bil"i*ty\, n.
   The condition or quality of being convertible; capability of
   being exchanged; convertibleness.

         The mutual convertibility of land into money, and of
         money into land.                         --Burke.

Source : WordNet®

convertibility
     n : the quality of being exchangeable (especially the ability to
         convert a currency into gold or other currencies without
         restriction) [ant: {inconvertibility}]