1. The provision of radiance to make something visible or bright, or the fact of being lit up: Turning on only three of the five ceiling spotlights provided just enough illumination for the pianist to perform his concert.
2. The amount or strength of effulgence available in a place or for a purpose: In the display case at the museum, the illumination was adjusted so as not to harm the artifacts by exposing them to too much bright shine.
3. The application and distribution of a beam to a subject: The scientist asked the students to bring the illumination closer to the microscope so the microbes could be seen easier.
4. The process of clarifying or explaining something: The volunteer guide at the gallery offered an exceptional illumination about the history of the Arctic artists and their work.
5. Intellectual or spiritual enlightenment: The pilgrim experienced a sense of illumination after spending a week with a religious retreat in the mountains.
6. A colored letter, design, or illustration decorating a manuscript or page, or the art or act of decorating written texts: The monks used gold leaf for part of the illumination of the religious texts they were carefully copying.
7. In medicine, throwing a beam on the body or a part or into a cavity for diagnostic purposes or shedding luster on an object under a microscope: The surgeon used a fiber optic instrument to bring illumination to the open chest surgery so he could see any possible blood clots.

The transmission of luminousness to an object by the use of clusters of very thin threadlike filaments of such substances as glass or plastic: The fiber-illumination used to incandesce the room produced a soft almost hazy shimmer, glowing through the fine strands.