IMPERCEPTIBLY

imperceptibly, unnoticeably

(adverb) in an imperceptible manner or to an imperceptible degree; “the power of the Secretary of State in London increased gradually but imperceptibly”

Source: WordNet® 3.1

Etymology

Adverb

imperceptibly (comparative more imperceptibly, superlative most imperceptibly)

Not noticeably; too small to be detected; too little to be perceived.

Source: Wiktionary

IMPERCEPTIBLE

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