Illumination in the most narrow definition is a manuscript adorned or embellished with paints, inks, gold and silver paints or decorations. This art form often gave a heavenly glow to figures represented in the manuscripts. However, today any decorated manuscript is referred to illumination.Illumination is not confined to the 13th century as most surviving art and manuscripts from the middle ages are examples of illumination. You may have seen examples in your own bible, when the first letter of the beginning word of a chapter is written in calligraphy and ornate designs.
Here is the entire alphabet made up from original Illuminated Letters.