Conceivable, a.
Etymology: [Cf. F. concevable.]
Capable of being conceived, imagined, or understood. "Any conceivable weight." Bp. Wilkins. 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.
Shades of meaning
Fernald has no entry of its own for conceivable; he lists it under likely.
Where he writes about it:
A thing is conceivable of which the mind can entertain the possibility; a thing is conjectural which is conjectured as possible or probable without other support than a conjecture, or tentative judgment; a thing is presumable which, from what is antecedently known, may betaken for granted in advance of proof. — from likely