Webster's English Dictionary

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imperceptible

im.per.cep.ti.ble \.im-p*r-'sep-t*-b*l\ \-ble-\ aj [MF, fr. ML 
   imperceptibilis, fr. L in- + LL perceptibilis per]ceptible : not 
   perceptible by a sense or by the mind : extremely slight, gradual , or 
   subtle - im.per.cep.ti.bly av