What does the root lum as in illuminate mean?

The root "lum" in "illuminate" comes from the Latin word "lūmen," which means "light" or "brightness."

Here's how the word evolved:

* Latin: *lūmen* (light)

* Late Latin: *illuminare* (to light up)

* Middle English: *illuminate* (to light up)

* Modern English: *illuminate* (to light up, to make clear, to enlighten)

So, "illuminate" literally means "to make something light." This meaning is extended to encompass the act of making something clear or understandable, hence its use in contexts like "illuminating the facts" or "illuminating a topic."