Traditional ArtDefinition of the Medium
Gouache is similar to watercolor and acrylic. But it has unique characteristics that differentiate it from both of them.
Like watercolor, it's water soluble and its binder is gum arabic (M. Graham & Co use honey ιn their gouache). The pigment ratio however is much higher compared to watercolor. It's also opaque (containing chalk). However, it can be thinned down to washes and worked just like watercolor. In that form it's less translucent than watercolors but you'll have a hard time to tell the difference. Most importantly it can build up opaque layers.
When it dries - like acrylic - the lights tend to be a bit darker and the darks a bit lighter. It dries fast and flat. Although - unlike acrylic - the opaque layers of gouache can be reactivated with water. Despite the matte look, gouache has a special vibrancy in opaque form due to the high pigment load.
If you work in watercolor, y