The Collaborative International Dictionary of English v.0.48:

Indiscernible \In`dis*cern"i*ble\, a. [Pref. in- not +
   discernible: cf. F. indiscernable.]
   Not to be discerned; imperceptible; not discoverable or
   visible.
   [1913 Webster]

         Secret and indiscernible ways.           --Jer. Taylor.
   -- In`dis*cern"i*ble*ness, n. -- In`dis*cern"i*bly, adv.
   Indiscerpibility

WordNet (r) 3.0 (2006):

indiscernible
    adj 1: difficult or impossible to perceive or discern; "an
           indiscernible increase in temperature" [ant:
           discernable, discernible]
    2: barely able to be perceived; "the transition was almost
       indiscernible"; "an almost insensible change" [syn:
       indiscernible, insensible, undetectable]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "indiscernible":
   alike, atomic, behind the curtain, behind the scenes, camouflaged,
   concealed, corpuscular, disguised, embryonic, evanescent, germinal,
   granular, hidden, impalpable, imperceptible, imponderable,
   inappreciable, indistinct, indistinctive, indistinguishable,
   infinitesimal, insensible, intangible, interchangeable, invisible,
   latent, microcosmic, microscopic, molecular, out of sight, secret,
   sightless, standard, stereotyped, subatomic, submerged, tenuous,
   thin, ultramicroscopic, unapparent, unappreciable, unbeheld,
   unbeholdable, undifferentiated, undiscernible, undiscriminated,
   undistinguishable, undistinguished, uniform, unnoticed,
   unobservable, unobserved, unperceivable, unperceived, unrealized,
   unseeable, unseen, unviewed, unwitnessed, viewless,
   without distinction