[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

imperceptibly
     adv : in an imperceptible manner or to an imperceptible degree;
           "the power of the secretary of state in london
           increased gradually but imperceptibly" syn: unnoticeably,
            observably ant: perceptibly
see also:
unnoticeably observably perceptibly 

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