Wordnet 3.0
ADJECTIVE (1)
1. 
 not capable of being distinguished or differentiated; 
- Example: "the two specimens are actually different from each other but the differences are almost indistinguishable"- Example: "the twins were indistinguishable"- Example: "a colorless person quite indistinguishable from the colorless mass of humanity"[syn: indistinguishable, undistinguishable]
The Collaborative International Dictionary of English v.0.48:
Undistinguishable \Undistinguishable\
   See distinguishable.
WordNet (r) 3.0 (2006):
undistinguishable
    adj 1: not capable of being distinguished or differentiated;
           "the two specimens are actually different from each other
           but the differences are almost indistinguishable"; "the
           twins were indistinguishable"; "a colorless person quite
           indistinguishable from the colorless mass of humanity"
           [syn: indistinguishable, undistinguishable] [ant:
           distinguishable]