Source : Webster's Revised Unabridged Dictionary (1913)

Gouache \Gouache\, n. [F., It. guazzo.]
   A method of painting with opaque colors, which have been
   ground in water and mingled with a preparation of gum; also,
   a picture thus painted.

Source : WordNet®

gouache
     n 1: an opaque watercolor prepared with gum
     2: a watercolor executed with opaque watercolors mixed with gum