The Collaborative International Dictionary of English v.0.48:

Conjectural \Con*jec"tur*al\, a. [L. conjecturalis: cf. F.
   conjectural.]
   Dependent on conjecture; fancied; imagined; guessed at;
   undetermined; doubtful.
   [1913 Webster]

         And mak'st conjectural fears to come into me. --Shak.
   [1913 Webster]

         A slight expense of conjectural analogy. --Hugh Miller.
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         Who or what such editor may be, must remain
         conjectural.                             --Carlyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

conjectural
    adj 1: based primarily on surmise rather than adequate evidence;
           "theories about the extinction of dinosaurs are still
           highly conjectural"; "the supposed reason for his
           absence"; "suppositious reconstructions of dead
           languages"; "hypothetical situation" [syn: conjectural,
           divinatory, hypothetical, hypothetic, supposed,
           suppositional, suppositious, supposititious]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "conjectural":
   abstract, academic, arguable, armchair, at issue, confutable,
   contestable, controversial, controvertible, debatable, deniable,
   disputable, doubtable, doubtful, dubious, dubitable, hypothetic,
   hypothetical, ideal, iffy, impractical, in dispute, in doubt,
   in dubio, in question, mistakable, moot, notional, open to doubt,
   open to question, postulatory, problematic, putative, questionable,
   refutable, reputed, speculative, suppositional, supposititious,
   suppositive, suppository, suspect, suspicious, theoretical