Source : Webster's Revised Unabridged Dictionary (1913)

Indistinguishable \In`dis*tin"guish*a*ble\, a.
   Not distinguishable; not capable of being perceived, known,
   or discriminated as separate and distinct; hence, not capable
   of being perceived or known; as, in the distance the flagship
   was indisguishable; the two copies were indisguishable in
   form or color; the difference between them was
   indisguishable.

Source : WordNet®

indistinguishable
     adj 1: exactly alike; incapable of being perceived as different;
            "rows of identical houses"; "cars identical except for
            their license plates"; "they wore indistinguishable
            hats" [syn: {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 colorless person quite
        indistinguishable from the colorless mass of humanity"
        [syn: {undistinguishable}] [ant: {distinguishable}]