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impossible to perceive:

(微小なために) «…に» 感じられない, 感知されない, 気づかれない(ほどわずかな) «to»

e.g. his head moved in an almost imperceptible nod.

imperceptibility |ˌimpərˌseptəˈbilədē| noun

late Middle English: from French, or from medieval Latin imperceptibilis, from in- ‘not’ + perceptibilis, from the verb percipere (see perceive).