The Collaborative International Dictionary of English v.0.48:

Imperceptible \Im`per*cep"ti*ble\, 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`per*cep"ti*ble*ness, n. -- Im`per*cep"ti*bly, adv.
   [1913 Webster]

         Their . . . subtilty and imperceptibleness. --Sir M.
                                                  Hale.
   [1913 Webster]

WordNet (r) 3.0 (2006):

imperceptible
    adj 1: 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: imperceptible, unperceivable] [ant:
           perceptible]

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