Meaning

verb make lighter or brighter

verb make free from confusion or ambiguity; make clear

verb add embellishments and paintings to (medieval manuscripts)

originFrom Middle English illuminaten, borrowed from Latin illūminātus, perfect passive participle of illūminō (“lighten, light up, show off”) (see -ate (verb-forming suffix) for more), from in + lūminō (“light up”), from lūmen (“light”). Cognate with Old English lȳman (“to glow, shine”). More at leam.

Etymology from Wiktionary, CC BY-SA 3.0