Other forms: illumined; illumines; illumining
To illumine is to shine a light on something, literally or figuratively. Your desk lamp might illumine the page of your book, while the words you're reading illumine your mind.
This literary term is a more poetic way of saying "illuminate," though its original meaning was the figurative "enlighten spiritually." It's a graceful verb to use when you're talking about light that shines or glows: "The dancing candlelight cheerfully illumines the dim room." It's also useful for describing ideas that spark an intellectual or spiritual understanding: "It's the first time I've read philosophy that truly illumines my perception."