The Collaborative International Dictionary of English v.0.48:

Imaginable \Im*ag"i*na*ble\, a. [L. imaginabilis: cf. F.
   imaginable.]
   Capable of being imagined; conceivable.
   [1913 Webster]

         Men sunk into the greatest darkness imaginable.
                                                  --Tillotson.
   -- Im*ag"i*na*ble*ness, n. -- Im*ag"i*na*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

imaginable
    adj 1: capable of being imagined; "that is one possible answer"
           [syn: conceivable, imaginable]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "imaginable":
   assumable, cogitable, conceivable, conceivably possible,
   conjecturable, contingent, fanciable, humanly possible, likely,
   plausible, possible, potential, premissable, presumable, probable,
   supposable, surmisable, thinkable