Illumination, from the Latin illuminare, 'to enlighten or illuminate', is the embellishment of a manuscript with luminous colours (especially gold and silver). In the past, the colouring of maps and prints was also called illumination. A miniature is sometimes referred to as an illumination.
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).
Raymond Clemens and Timothy Graham, Introduction to Manuscript Studies (Ithaca, NY: Cornell University Press, 2007), 33-34.