The Collaborative International Dictionary of English v.0.48:

Impasto \Im*pas"to\, n. [It. See Impaste.] (Paint.)
   The thickness of the layer or body of pigment applied by the
   painter to his canvas with especial reference to the
   juxtaposition of different colors and tints in forming a
   harmonious whole. --Fairholt.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impasto
    n 1: painting that applies the pigment thickly so that brush or
         palette knife marks are visible