Conjectural, a.
Etymology: [L. conjecturalis: cf. F. conjectural.]
Dependent on conjecture; fancied; imagined; guessed at; undetermined; doubtful. And mak'st conjectural fears to come into me. Shak. A slight expense of conjectural analogy. Hugh Miller. Who or what such editor may be, must remain conjectural. Carlyle.
Shades of meaning
Fernald has no entry of its own for conjectural; 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