The term “illumination” originally denoted the embellishment of the text of handwritten books with gold or, more rarely, silver, giving the impression that the page had been literally illuminated. Though various Islamic societies practiced this art, Europe had the longest tradition of illuminating manuscripts.
The art was at its height height during the middle ages. Within scriptoria or workshops where books were copied and hand written there was a differentiation between those who “historiated” (i.e., illustrated texts by relevant paintings) and those who “illuminated” (i.e., supplied the decorative work that embellished initial capital letters and often spilled into margins and borders.
Among the most highly developed illuminated text were those of the Celts. Christians encountering the Celts drew from the indigenous culture. Celtic tradition seeped into the roots of Christianity as they were laid down in the Islands. Literate "historic" Christian culture was superimposed onto oral "prehistoric" Celtic culture. The traditions of the two merged and complemented other in many ways one of which was the illuminated manuscripts produced in early Christian Ireland and Britain.
Early Insular manuscripts are incredible documents of the transition from an oral to a literate culture. The highly developed visual sense of the Celts was adapted to Christian texts with stunning results that affected manuscript illumination for hundreds of years in Europe.
Top: Illuminated Manuscript Koran, The right side of an illuminated double-page frontispiece, Walters Art Museum Ms.
Middle: From the book of Mark from the book of Kells medieval illuminated manuscript Irish Celtic knotwork
Bottom: example of latter European Manuscript
compiled from several sources