Opaque Paints
Opaque paints (gouache paints, body colours), paints that more or less completely obscure the colour of the surface on which they are painted (see gouache painting), as opposed to Glazes.
Opaque artists‘ acrylic paints and artists‘ oils are designated as opaque paints by a black square on the paint tube.
Source: Meyers Großes Konversations-Lexikon, 6. Auflage 1905–1909