The Collaborative International Dictionary of English v.0.48:

achievability \achievability\ n.
   1. 1 the possibility of being achieved or accomplished; -- a
      property which may be possessed by a contemplated act.

   Syn: attainability, attainableness
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

achievability
    n 1: the state of being achievable [syn: achievability,
         attainability, attainableness]