Also referred to as bodycolor and opaque watercolor, the term "gouaches" was first used during the eighteenth century in France to describe the use of a translucent water-based paint that had been rendered opaque by the addition of white pigment or chalk bound together with a binding agent such as gum arabic.

Contrary to watercolor's key characteristic of transparent luminosity, gouache is defined by its matte and opaque quality.

Today's commercially available product known as gouache differs considerably from that used by earlier draftsmen. (Louis-Gabriel Moreau, Parc de Saint-Cloud, 1955.188; Henri de Toulouse-Lautrec, Trapeze at the Medrano Circus, 1934.34).

(thanks to artmuseums.harvard.edu/sargentatharvard/drawingglossary.html)