The Collaborative International Dictionary of English v.0.48:

Conceivable \Con*ceiv"a*ble\, a. [Cf. F. concevable.]
   Capable of being conceived, imagined, or understood. "Any
   conceivable weight." --Bp. Wilkins.
   [1913 Webster]

         It is not conceivable that it should be indeed that
         very person whose shape and voice it assumed.
                                                  --Atterbury.
   -- Con*ceiv"a*ble*ness, n. -- Con*ceiv"a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

conceivableness
    n 1: the state of being conceivable [syn: conceivableness,
         conceivability]