Gesso is the Italian word for the white mineral gypsum. It is used as a ground or preparatory layer to ensure a smooth surface for painting or gilding on wood. It was also sometimes used for the priming of canvas.
Gesso consists of gypsum bound in size (an adhesive made from animal skin and waste). The common practice was for a coarse lower layer to be applied (gesso grosso), followed by a finer upper layer (gesso sottile). It has to be scraped and sanded to make it absolutely smooth. In some unfinished or damaged pictures parts of the gesso layer can be seen.
The gesso could be built up to form a low relief on the wood known as pastiglia. It could also be cut or incised - much of the fine details of picture frames is cut in the gesso rather than in the wood below. In panel painting north of the Alps, instead of gypsum chalk or some other form of calcium carbonate was generally used bound with size as the white ground.