Subliminal
Part of speech: adjective
Definition: Below the threshold of conscious perception, especially if still able to produce a response.
Example sentence: Our subliminal mental processes operate outside awareness because they arise in these portions of our mind that are inaccessible to our conscious self; their inaccessibility is due to the architecture of the brain rather than because they have been subject to Freudian motivational forces like repression.
Imperceptible
Part of speech: adjective
Definition: Not perceptible, not detectable, too small in magnitude to be observed.