The Collaborative International Dictionary of English v.0.48:

Possible \Pos"si*ble\, a. [F., fr. L. possibilis, fr. posse to
   be able, to have power; potis able, capable + esse to be. See
   Potent, Am, and cf. Host a landlord.]
   Capable of existing or occurring, or of being conceived or
   thought of; able to happen; capable of being done; not
   contrary to the nature of things; -- sometimes used to
   express extreme improbability; barely able to be, or to come
   to pass; as, possibly he is honest, as it is possible that
   Judas meant no wrong.
   [1913 Webster]

         With God all things are possible.        --Matt. xix.
                                                  26.
   [1913 Webster]

   Syn: Practicable; likely. See Practicable.
        [1913 Webster]

WordNet (r) 3.0 (2006):

possible
    adj 1: capable of happening or existing; "a breakthrough may be
           possible next year"; "anything is possible"; "warned of
           possible consequences" [ant: impossible]
    2: existing in possibility; "a potential problem"; "possible
       uses of nuclear power" [syn: potential, possible] [ant:
       actual, existent]
    n 1: something that can be done; "politics is the art of the
         possible"
    2: an applicant who might be suitable

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "possible":
   accomplishable, achievable, admissible, advisable, algorismic,
   algorithmic, aliquot, attainable, available, between the lines,
   cardinal, cogitable, conceivable, conceivably possible, contingent,
   covert, credible, cryptic, decimal, delitescent, differential,
   digital, doable, dormant, earthly, esoteric, even, expedient,
   exponential, feasible, figural, figurate, figurative, finite,
   fractional, hibernating, hidden, humanly possible, imaginable,
   imaginary, impair, impossible, infinite, integral, irrational,
   latent, likely, logarithmic, logometric, lurking, mortal, muffled,
   mystic, negative, numeral, numerary, numerative, numeric,
   obfuscated, obscured, occult, odd, on, ordinal, pair, plausible,
   positive, potential, practicable, practical, prime, probable,
   radical, rational, reachable, real, realizable, reasonable,
   reciprocal, sleeping, submerged, submultiple, surd, tenable,
   thinkable, transcendental, under the surface, underlying,
   unmanifested, veiled, viable, virtual, workable