Refers to a soft, plastic material that can be spread or daubed on a wall, ceiling, or other surface, where it afterwards hardens. In the context of art and architecture,
it generally refers specifically to a mixture of water, lime, and sand, often combined with other materials, such as animal hair, to give the resulting material strength, texture, and if the surface is to be painted, porosity.

Source: AAT 

http://vocab.getty.edu/page/aat/300014922