Wordnet 3.0
ADJECTIVE (1)
1. 
 not capable of being precisely or readily described; 
 not easily put into words; 
- Example: "an indefinable feeling of terror"- Example: "an abstract concept that seems indefinable"[syn: indefinable, undefinable]
The Collaborative International Dictionary of English v.0.48:
Undefinable \Undefinable\
   See definable.
WordNet (r) 3.0 (2006):
undefinable
    adj 1: not capable of being precisely or readily described; not
           easily put into words; "an indefinable feeling of
           terror"; "an abstract concept that seems indefinable"
           [syn: indefinable, undefinable]