im•per•cep•ti•blePronunciation: (im"pur-sep'tu-bul), [key]
very slight, gradual, or subtle: the imperceptible slope of the road.
not perceptible; not perceived by or affecting the senses.
something not capable of being perceived by the senses: metaphysical speculation about imperceptibles.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.