A thin, transparent glaze of color applied to a ground before painting on it. This layer may be applied below or on top of an underdrawing layer. Thin preparatory layer not only reduces absorbency but can also be used as a middle tone for the painting.

Source: Looking at Paintings: A Guide to Technical Terms / eds. Tiarna Doherty and Anne T Woollett. (Los Angeles: J. Paul Getty Museum, 2009), p. 42.