imperceptible
im-per-cep-ti-ble
adjective
Too slight, gradual, or subtle to be noticed or felt.
"The change in light was almost imperceptible."
Thesaurus
Synonyms: undetectable, unperceivable, insensible, subliminal, incognizable, indiscernible, impalpable, unobservable, incognoscible
Antonyms: perceptible