Imperceptible
Pronunciation : Im`per*cep"ti*ble
Part of Speech : a.
Etymology : [Pref. im- not + perceptible: cf. F. imperceptible.]
Definition : Defn: Not perceptible; not to be apprehended or cognized by the souses; not discernible by the mind; not easily apprehended. Almost imperceptible to the touch. Dryden. Its operation is slow, and in some cases almost imperceptible. Burke. -- Im`per*cep"ti*ble*ness, n. -- Im`per*cep"ti*bly, adv. Their . . . subility and imperceptibleness. Sir M. Hale.
Source : Webster's Unabridged Dictionary, 1913