The Collaborative International Dictionary of English v.0.48:

Indistinguishable \In`dis*tin"guish*a*ble\
   ([i^]n`d[i^]s*t[i^][ng]"gw[i^]sh*[.a]*b'l), 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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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, indistinguishable]
    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:
       indistinguishable, undistinguishable] [ant:
       distinguishable]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "indistinguishable":
   alike, all one, all the same, blear, bleared, bleary, blurred,
   blurry, confused, consubstantial, dark, dim, duplicate, equal,
   equivalent, exactly alike, faint, feeble, filmy, foggy, fuzzy,
   half-seen, half-visible, hazy, homoousian, identic, identical,
   ill-defined, imperceptible, inconspicuous, indefinite,
   indiscernible, indistinct, indistinctive, inseparable,
   interchangeable, just alike, like, low-profile, merely glimpsed,
   misty, obscure, one, out of focus, pale, same, selfsame,
   semivisible, shadowy, standard, stereotyped, tantamount, twin,
   uncertain, unclear, undefined, undifferent, undifferentiated,
   undiscriminated, undistinguishable, undistinguished, uniform,
   unplain, unrecognizable, vague, weak, without difference,
   without distinction