What does "imperceptible" mean?

Imperceptible: 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.

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