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
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)