Source: Oxford University Press
Pigment dissolved in water and bound by a colloid agent so that it adheres to the working surface when applied with the brush. The same name is used for a work of art in that medium. Watercolour may be transparent or opaque and is usually applied to paper, but sometimes also to such materials as silk or vellum. The term arises because, in varying degrees, water is always used in the largest proportion and, in the purest application of the medium, twice—both to mix pigments and to dilute the colours. This article concentrates on the use of watercolour predominantly in Europe and the United Kingdom; for further discussion see Indian subcontinent, §V, 4(ix) and Islamic art, §I, 8.
From Grove Art Online