WordWeb Online

1. Exactly alike; incapable of being perceived as different
   "they wore indistinguishable hats"
   - identical
    
2. 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 colourless person quite indistinguishable from the colourless mass of humanity"
   - undistinguishable

See also: indistinguishability, same

Antonym: distinguishable