Gouache is a water-based paint similar to watercolor. Where watercolors are transparent, allowing for subtle layering, gouache mixes colored pigments with an opaque white pigment, often chalk, creating a velvety matte finish that absorbs light rather than reflecting it. Gouache-style paints have been in use since at least the 9th century, often appearing in Persian miniatures. By the 14th century, trade routes had spread gouache to Europe, where it’s crisp graphic appearance, ability to re-wet, and relatively cheap production made it a staple of commercial illustration and early art education.
Reed Enger, "Gouache, From the Italian word for 'mud'," in Obelisk Art History, Published January 27, 2015; last modified July 20, 2019, http://arthistoryproject.com/mediums/gouache/.