Source : Webster's Revised Unabridged Dictionary (1913)

Improvable \Im*prov"a*ble\, a. [From {Improve}.]
   1. Capable of being improved; susceptible of improvement;
      admitting of being made better; capable of cultivation, or
      of being advanced in good qualities.

            Man is accommodated with moral principles,
            improvable by the exercise of his faculties. --Sir
                                                  M. Hale.

            I have a fine spread of improvable lands. --Addison.

   2. Capable of being used to advantage; profitable;
      serviceable; advantageous.

            The essays of weaker heads afford improvable hints
            to better.                            --Sir T.
                                                  Browne.
      -- {Im*pro"a*ble*ness}, n. -- {Im*prov"a*bly}, adv.

Source : WordNet®

improvable
     adj : susceptible of improvement