[1] : The Collaborative International Dictionary of English v.0.48

imperceptible \im`percep"tible\, a. pref. im- not +
   perceptible: cf. f. imperceptible.
   not perceptible; not to be apprehended or cognized by the
   senses; not discernible by the mind; not easily apprehended.
   1913 webster

         almost imperceptible to the touch.       --dryden.
   1913 webster

         its operation is slow, and in some cases almost
         imperceptible.                           --burke.
   -- im`percep"tibleness, n. -- im`percep"tibly, adv.
   1913 webster

         their . . . subtilty and imperceptibleness. --sir m.
                                                  hale.
   1913 webster
see also:
im`percep"tibleness im`percep"tibly 

[2] : WordNet (r) 2.0

imperceptible
     adj : impossible or difficult to perceive by the mind or senses;
           "an imperceptible drop in temperature"; "an
           imperceptible nod"; "color is unperceivable to the
           touch" syn: unperceivable ant: perceptible
see also:
unperceivable perceptible 

[3] : Moby Thesaurus II by Grady Ward, 1.0

70 moby thesaurus words for "imperceptible":
   atomic, behind the curtain, behind the scenes, camouflaged,
   concealed, corpuscular, disguised, embryonic, ephemeral,
   evanescent, faint, fugitive, germinal, granular, hidden,
   ill-defined, impalpable, imponderable, inappreciable, inaudible,
   inconsiderable, inconspicuous, indiscernible, indistinct,
   indistinguishable, infinitesimal, insensible, insignificant,
   intangible, invisible, latent, microcosmic, microscopic, minuscule,
   minute, molecular, momentary, obscure, out of sight, secret,
   sightless, slight, subatomic, submerged, subtle, tenuous, thin,
   tiny, trivial, ultramicroscopic, unapparent, unappreciable,
   unbeheld, unbeholdable, unclear, undiscernible, undistinguishable,
   unnoticeable, unnoticed, unobservable, unobserved, unperceivable,
   unperceived, unrealized, unseeable, unseen, unviewed, unwitnessed,
   vague, viewless

Imperceptible >> I Words
Imperceptible, definition of term: Imperceptible
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