/ ˌɪmpəˈseptəbl; NAmE -pərˈs-/ adjective
very small and therefore unable to be seen or felt;
OPP perceptible :
imperceptible changes in temperature
► im·per·cept·ibly / -əbli; NAmE / adverb
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WORD ORIGIN
late Middle English : from French , or from medieval Latin imperceptibilis , from in- not + perceptibilis , from the verb percipere seize, understand, from per- entirely + capere take.