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):

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

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "conceivable":
   apparent, appreciable, apprehensible, ascertainable, believable,
   cogitable, cognizable, colorable, comprehensible,
   conceivably possible, contingent, credible, discernible,
   discoverable, distinguishable, earthly, fanciable, fiduciary,
   graspable, humanly possible, imaginable, knowable, likely, mortal,
   perceptible, plausible, possible, potential, prehensible, probable,
   reasonable, recognizable, reliable, seizable, supposable, tenable,
   thinkable, trustworthy, trusty, understandable, unexceptionable,
   unimpeachable, unquestionable, worthy of faith