Grisaille, from the French word gris meaning grey, is a term used to describe works of art painted entirely in a monochromatic palette. Technically speaking, there are other terms that apply when the monochromatic palette used is of a different color (brunaille for brown, verdaille for green, for instance), but grisaille is often misused to cover all monochrome works, regardless of hue. There are also plenty of works that are considered grisaille that are not perfectly, strictly speaking, in just one color, but the palette is severely curtailed.
Works “en grisaille” as they are usually referred to can be done as finished works, but they are also used to mimic the three-dimensional effect of sculpture in a tromp l’oeil style, to provide a basis for adaptation by engravers or illustrators, or to “rough in” an oil painting’s structure.
In the world of antiques, grisaille is often seen in connection with the decoration on Chinese export porcelain (sometimes the entire work is decorated en grisaille, sometimes only a portion), and the value there is derived from age and condition, but works executed completely en grisaille do frequently see a bump in value. In terms of fine art, the highest prices are reserved for early artists like Van Dyck who were known for their use of the technique, but grisaille works are, in general, popular and appealing because it’s generally conceded that working with a full palette can hide some weakness in skill and execution, whereas such a limited palette requires a more skillful hand.