A method of painting solely in shades of monochrome grey or another neutral colour. In the Renaissance and later grisaille was used for paintings, often those imitating the appearance of relief sculpture. It was also sometimes employed for underpainting, or for sketches (for example the oil sketches of Rubens). Grisaille enjoyed a revival of popularity among French 18th-century artists such as François Boucher.
Text source: The Oxford Concise Dictionary of Art Terms (2nd Edition) by Michael Clarke