Wordnet 3.0
NOUN (2)
1. 
 an opaque watercolor prepared with gum; 
2. 
 a watercolor executed with opaque watercolors mixed with gum; 
The Collaborative International Dictionary of English v.0.48:
Gouache \Gouache\ (gw[.a]sh), 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.
   [1913 Webster]
WordNet (r) 3.0 (2006):
gouache
    n 1: an opaque watercolor prepared with gum
    2: a watercolor executed with opaque watercolors mixed with gum