Food disguised to look like other food, or some other object altogether. A modern analogy for a subtleties would be character cakes, but in period they were either sweet or savoury.
A favourite example of a subtelty was skinning a peacock (keeping the feathers in tact), roasting the bird, and then replacing the skin back over it so it looked like the live thing. Other common subtelty creations are dragons, castles, birds, heraldic devices.
Also note that there are about 15 different ways of spelling subtelty.
http://groups.yahoo.com/group/SCA_Subtleties/ This list is a forum for (but not restricted to) SCA members dedicated to the research and recreation of subtleties (also reffered to as sotelties, soltelties, etc.), entremets and illusion foods within SCA timeframe. We study documented recipes and examples of this food art from the Roman, Medieval and Renaissance sources as well as sharing our attempts at recreating our own versions.
Entremets, illusion foods and subtleties links http://moas.atlantia.sca.org/wsnlinks/index.php?action=displaycat&catid=66
Gode Cookery Illusion Foods http://www.godecookery.com/illusion/illusion.html
Illusion Dishes http://members.aol.com/renfrowcm/illusion.html
Incredible Foods, Sotelties and Entremets http://www.godecookery.com/incrd/incrd.htm
Subtleties Entremets and Illusion Foods http://www.florilegium.org/files/FOOD-SWEETS/entertaing-fds-art.html
Atlantian A&S Links: Entremets, Illusion Foods, & Subtleties http://moas.atlantia.sca.org/wsnlinks/index.php?action=displaycat&catid=66