1. The art of illumination on vellum, ivory, etc, using pigment in an aqueous gum binding medium.
As quoted from: Jo Kirby, Susie Nash, and Joanna Cannon, eds. 'the appendix: Pigment Glossary', "Limning", in Trade in Artists’ Materials: Markets and Commerce in Europe to 1700. (London: Archetype Publications, 2010), 452. 452.
Oxford English Dictionary definitions:
1. To illuminate (letters, manuscripts, books).
2. To adorn or embellish with gold or bright colour; to depict in (gold, etc.). Also (rare), to lay on (colour).
3. To paint (a picture or portrait); to portray, depict (a subject). Formerly spec. to paint in water-colour or distemper.
4. Prov. to limn the water , limn (something) on water : said of something transient or futile.
5. Absol. or intr. To paint; esp. to paint in water-colour or distemper.
As quoted from: http://www.oed.com/view/Entry/108501?rskey=4R4g8S&result=1&isAdvanced=fa...