The Collaborative International Dictionary of English v.0.48:

Thinkable \Think"a*ble\, a.
   Capable of being thought or conceived; cogitable. --Sir W.
   Hamilton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thinkable
    adj 1: capable of being conceived or imagined or considered
           [ant: unthinkable]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "thinkable":
   believable, cogitable, conceivable, conceivably possible,
   contingent, credible, fanciable, feasible, humanly possible,
   imaginable, likely, plausible, possible, potential, probable,
   reasonable, supposable, tenable