Wikipedia defines it more fully: “similar to watercolor in that it can be re-wetted, it dries to a matte finish, and the paint can become infused with its paper support. It is similar to acrylic or oil paints in that it is normally used in an opaque painting style and it can form a superficial layer. Many manufacturers of watercolor paints also produce gouache and the two can easily be used together.” Basically, it is like an opaque watercolor paint.