Imperceptible
IMPERCEP'TIBLE, adjective
1. Not to be perceived; not to be known or discovered by the senses. We say a thing is imperceptible to the touch, to the eye or sight, to the ear, to the taste or smell. Hence,
2. Very small; fine; minute in dimensions; or very slow in motion or progress; as, the growth of a plant or animal is imperceptible; it is too slow to be perceived by the eye.
IMPERCEP'TIBLE, noun That which cannot be perceived by the senses on account of its smallness. [Little Used.]