Definition of imperceptible adjective from the Oxford Advanced American Dictionary

 

imperceptible

 adjective
adjective
NAmE//ˌɪmpərˈsɛptəbl//
 
 
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very small and therefore unable to be seen or felt imperceptible changes in temperature The differences were imperceptible to all but the most trained eye. There was an almost imperceptible pause as she gathered her breath to speak. opposite perceptible
imperceptibly
 
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NAmE//ˌɪmpərˈsɛptəbli//
 
adverb The daylight faded almost imperceptibly into night.
See the Oxford Advanced Learner's Dictionary entry: imperceptible