The art of decorating manuscripts with gold and silver. In the middle ages one of the dominant forms of art was document making usually priceless books. Each page was carefully written by scribes (usually monks) on parchment of animal hides.
The term is not just restricted to application of gold, as it has become the synonym for applying areas of colour and ornamentation to areas of a document. Also it is not restricted to a certain age. Some of the oldest examples are Egyptian scrolls on papyrus (e.g. the Book of the Dead). The Greek and Roman civilisation were also known for the art of illuminating.