Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

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