Grisaille is a monochrome painting executed entirely in different shades of gray or a grayish tone.
'Gris' is a french term for gray. Paintings in a brown scale are referred to as brunaille and paintings in a green scale are referred to as verdaille.
Grisaille was typically used during the Renaissance period as a stand alone painting technique to imitate sculpture. It can also be used as a preliminary painting stage, where the grisaille forms an underpainting which can then be glazed over.
In the 20th Century with the prominence of painting alla prima, the grisaille method is not commonly practised.