Not noticeably, too small to be detected, too little to be perceived.(adverb)
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Use "imperceptibly" in a sentence
"Highly dominant, moderately dominant, and what I call imperceptibly dominant or submissive."
"Clark smiled almost imperceptibly, that is his face expressed an inward amusement because a number of tiny lines wrinkled into being at the corners of his gray eyes, and his lips pushed out ever so slightly."
"Cornelius or Schnorr (imperceptibly, that is to say), and reminiscences are poured out slowly while the smoke puffs from the pipes."