A wash is a fluid, diluted application of pigment laid down evenly over an area — usually transparent, usually quick, and usually absorbed at least partly into the ground.
Washes are most associated with watercolour and ink, where the medium is thinned with water and floated onto absorbent paper, but oil and acrylic painters also lay washes (thinned with medium or water) as toned grounds or broad atmospheric passages. A wash reads as flat, even, and slightly stained rather than built up — the paper or canvas weave often shows through. Description distinguishes a flat wash (uniform colour) from a graded wash (smoothly transitioning in tone) and a variegated wash (multiple colours bleeding into one another while wet). Distinct from a glaze, which sits on top of a sealed under-layer rather than soaking in.