The Collaborative International Dictionary of English v.0.48:

Definable \De*fin"a*ble\, a. [From Define.]
   Capable of being defined, limited, or explained;
   determinable; describable by definition; ascertainable; as,
   definable limits; definable distinctions or regulations;
   definable words. -- De*fin"a*bly, adv.
   [1913 Webster]