The Collaborative International Dictionary of English v.0.48:

Soluble \Sol"u*ble\, a. [L. solubilis, fr. solvere, solutum, to
   loosen, to dissolve: cf. F. soluble. See Solve, and cf.
   Solvable.]
   1. Susceptible of being dissolved in a fluid; capable of
      solution; as, some substances are soluble in alcohol which
      are not soluble in water.
      [1913 Webster]

            Sugar is . . . soluble in water and fusible in fire.
                                                  --Arbuthnot.
      [1913 Webster]

   2. Susceptible of being solved; as, a soluble algebraic
      problem; susceptible of being disentangled, unraveled, or
      explained; as, the mystery is perhaps soluble. "More
      soluble is this knot." --Tennyson.
      [1913 Webster]

   3. Relaxed; open or readily opened. [R.] "The bowels must be
      kept soluble." --Dunglison.
      [1913 Webster]

   Soluble glass. (Chem.) See under Glass.
      [1913 Webster]

WordNet (r) 3.0 (2006):

soluble
    adj 1: (of a substance) capable of being dissolved in some
           solvent (usually water) [ant: indissoluble,
           insoluble]
    2: susceptible of solution or of being solved or explained; "the
       puzzle is soluble" [ant: insoluble]