Wordnet 3.0
ADJECTIVE (1)
1. 
 capable of being defined, limited, or explained; 
- Example: "definable terms"- Example: "definable rules"
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]
WordNet (r) 3.0 (2006):
definable
    adj 1: capable of being defined, limited, or explained;
           "definable terms"; "definable rules"