The Collaborative International Dictionary of English v.0.48:

Gesso \Ges"so\, n. [It., chalk, plaster.]
   1. Plaster of Paris, or gypsum, esp. as prepared for use in
      painting, or in making bas-reliefs and the like; by
      extension, a plasterlike or pasty material spread upon a
      surface to fit it for painting or gilding, or a surface so
      prepared.
      [Webster 1913 Suppl.]

   2. A work of art done in gesso. [Obs.]
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

gesso
    n 1: gypsum or plaster of Paris spread on a surface to make it
         suitable for painting or gilding (or a surface so prepared)