Illumination is light. Kids up past their bedtimes have been known to read entire books under their covers using only the illumination from a flashlight.
Use the noun illumination to talk about light, like the illumination of the moon on the surface of a lake. Illumination can also refer to an understanding or a spiritual awareness, like when a mystery of your faith suddenly makes sense to you. The Latin root is illuminationem, "throw into light" — you can picture the light bulb over a person's head in a cartoon to remember the various meaning of illumination.