IMPERCEPTIBLE

imperceptible, unperceivable

(adjective) impossible or difficult to perceive by the mind or senses; “an imperceptible drop in temperature”; “an imperceptible nod”; “color is unperceivable to the touch”

Source: WordNet® 3.1

Etymology

Adjective

imperceptible (comparative more imperceptible, superlative most imperceptible)

not perceptible, not detectable, too small in magnitude to be observed

Synonyms: imperceivable, undistinguishable, unperceivable

Antonyms: detectable, perceptible

Source: Wiktionary

Im`per*cep"ti*ble, a. Etym: [Pref. im- not + perceptible: cf. F. imperceptible.]

Definition: Not perceptible; not to be apprehended or cognized by the souses; not discernible by the mind; not easily apprehended. Almost imperceptible to the touch. Dryden. Its operation is slow, and in some cases almost imperceptible. Burke.

– Im`per*cep"ti*ble*ness, n.

– Im`per*cep"ti*bly, adv. Their . . . subility and imperceptibleness. Sir M. Hale.

Source: Webster’s Unabridged Dictionary 1913 Edition