Main Entry: im·per·cep·ti·ble

Pronunciation: \ˌim-pər-ˈsep-tə-bəl\

Function: adjective

Etymology: Middle English, from Middle French, from Medieval Latin imperceptibilis, from Latin in- + Late Latin perceptibilis perceptible

Date: 15th century

: not perceptible by a sense or by the mind : extremely slight, gradual, or subtle <imperceptible differences>

— im·per·cep·ti·bly \-ˈsep-tə-blē\ adverb