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.

adjective

apparent , conspicuous , distinct , evident , noticeable , obvious , perceptible , seeable , striking , unobscured

adjective

ephemeral , evanescent , fine , gradual , impalpable , imponderable , inappreciable , inaudible , inconsiderable , inconspicuous , indiscernible , indistinct , indistinguishable , infinitesimal , insensible , insignificant , invisible , microscopic , minute , momentary , shadowy , slight , small , subtle , tiny , trivial , undetectable , unnoticeable , vague , intangible , unobservable , faint , hidden , scant , secret , subliminal , unapparent , unnoticed