means The act of lighting something up, or the flash of sudden understanding that makes a once-murky idea clear.

from From Latin illuminare, 'to light up, throw light on' — built from in- ('upon') and lumen ('light'), the same lumen that glows in luminous and luminary. It arrived in English through Old French in the late Middle Ages, first carrying its literal sense of physical light, then expanding to the mental kind. The decorated, gold-leafed pictures in medieval manuscripts were called illuminations too — the page itself made to shine.