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]