Impasto is a technique used in painting where paint is laid on the canvas very thickly, usually thickly enough that the brush or painting-knife strokes are visible. Paint can also be mixed right on the canvas.
Impasto features strongly in many oil paintings, and is also possible, though rarely used, in acrylic paintings. It is not possible in watercolor or tempera, owing to the inherent thinness of these media.
Impasto was used by Rembrandt and Titian, both of whom often juxtaposed it with more delicate painting. It is more prominent in later works, however, Vincent van Gogh using it frequently. Still more recently, Frank Auerbach has used such heavy impasto that some of his paintings almost become three-dimensional.