imperceptible
1. [ adjective ] impossible or difficult to perceive by the mind or senses
Synonyms: unperceivable
Examples:

"an imperceptible drop in temperature" "an imperceptible nod" "color is unperceivable to the touch"

Related terms: perceptible indiscernible incognizable impalpable subliminal unobservable invisible inaudible impalpable
Similar spelling:   imperceptibly
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  imperative_mood