Impossible or difficult to perceive by the mind or senses: an imperceptible drop in temperature. (adjective)
So subtle, slight, or gradual as to be barely perceptible: an imperceptible nod. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "imperceptible" in a sentence
"My brother rose to do so but he saw neither ewer nor basin; yet his host kept washing his hands with invisible soap in imperceptible water and cried,"
"But [since without water tasting is impossible] either (a) we must suppose that water contains in itself [uniformly diffused through it] the various kinds of savour, already formed, though in amounts so small as to be imperceptible, which is the doctrine of"
"For what is called imperceptible and incomprehensible does not produce an impression, because it is not a real fact, but a concept: the requisite of bulk in the beautiful is thus reduced to the effective reality of the physical fact, which serves for the reproduction of the beautiful."