1. To make something visible or bright with effulgence, or be lit up: Esther sat near the window so the sunlight would illuminate her sewing better and she could then easily fasten the buttons on the shirt.
2. To decorate something with lamps for a celebration: The youth groups planned to illuminate the statues in the city park with bright lanterns in time for the holidays.
3. To make something clear, or easier to understand and to appreciate: Dr. Murphy's explanation was illuminating on the subject of portrait photography and so Jim was able to appreciate black and white portraits easier.

Footnotes illuminated the difficult definitions in the dictionary of scientific terms.

4. To add colored letters, illustrations, and designs to a manuscript or the borders of a page: In the ancient monasteries, the resident monks used their time and skills to illuminate manuscripts, enhancing the beauty of the words with the beauty of the pages.
5. To provide someone with knowledge or with intellectual or spiritual enlightenment: Mary Jo went on a spiritual retreat for the weekend and felt illuminated by the workshops and lectures she attended.

1. Informative and enlightening, often by revealing or emphasizing facts that were previously obscure or not comprehensible: The illuminating words from one of her colleagues made it possible for Rosetta to understand the crisis that had occurred at work.
2. Providing new or useful information, so something becomes clearer and easier to understand: The art history course Sue was taking in her seniors' development services proved to be most illuminating; especially, when there were also trips to the local art gallery.

An earlier name for kerosene: Illuminating oil was once suitable for burning in order to provide brightness in lamps, etc.

The general store in the village had two bottles of illuminating oil on the shelf, as well as lamps in which to burn it, to provide light in the cabin where Jane was staying during her vacation.