Noah Webster's New International Dictionary of the English Language

1. (v. t.) To shed light on; to brighten; to supply with light, literally or figuratively; as, to illuminate a passage in a book.

2. (v. t.) To light up; to decorate with artificial lights, as a building or city, in token of rejoicing or respect.

3. (v. t.) To adorn, as a book or page with borders, initial letters, or miniature pictures in colors and gold, as was done in manuscripts of the Middle Ages.

4. (v. t.) To make plain or clear; to dispel the obscurity to by knowledge or reason; to explain; to elucidate; as, to illuminate a text, a problem, or a duty.

5. (v. i.) To light up in token or rejoicing.

6. (a.) Enlightened.

7. (n.) One who enlightened; esp., a pretender to extraordinary light and knowledge.