Imprimatura: The first thin, transparent toning of the canvas.
An imprimatura brings the surface closer to a midtone, allowing the painter to more easily gauge colors and values than if he were to work on a pure white surface. An imprimatura can be generic, not specific to the subject being painted on top of it, or it can be specifically adapted to the the subject being painted.
Example: An earth red imprimatura would be more ideally suited for a landscape, while something browner and more neutral would be better suited for a setup in the studio.
Either way, the painter should be conscious of how the imprimatura is going to affect later stages in the painting. If an imprimatura is too dark and gray, it will drag down the luminosity of the colors laid on top of it.
Adrian Gottlieb
GottliebStudios.com/Classical_Glossary