Works done by painting on the front or the reverse of 
glass, whether or not the paint is then fired onto the 
glass. Includes works on flat surfaces, vessels, and other 
types of works. Painted glass mirrors and other glass 
decoration, and painted works in the manner of painting 
on canvas or wooden panels were known since ancient times. 
In the context of stained glass windows and panels, refers 
to parts of the image that are painted on glass (often 
stained glass), especially the smaller details.

http://vocab.getty.edu/page/aat/300343822