When a person perceives a stimulus and the stimulus is no longer the focus of their attention, it leaves an impression.
wˌɛn ɐ pˈɜːsən pɚsˈiːvz ɐ stˈɪmjʊləs ænd ðə stˈɪmjʊləs ɪz nˌoʊ lˈɑːŋɡɚ ðə fˈoʊkəs ʌv ðɛɹ ɐtˈɛnʃən, ɪt lˈiːvz ɐn ɪmpɹˈɛʃən.