Illuminated manuscripts are hand-written books, generally on parchment or vellum, with illustrations, which were produced during the Middle Ages, before the invention of printing.
The illustrations (illuminations) could be just an initial capital letter, a whole page, or part of a whole page. They were often very elaborate, using gold leaf.
Many of the greatest illuminated manuscripts were produced in Paris and Burgundy in the 14th century; 'Les Tres Riches Heures du Duc du Berry' by the Limbourg brothers is one of the most splendid (Chantilly, Musee Conde).