The Collaborative International Dictionary of English v.0.48:

Demitint \Dem"i*tint`\, n. (Fine Arts)
   (a) That part of a painting, engraving, or the like, which is
       neither in full darkness nor full light.
   (b) The shade itself; neither the darkest nor the lightest in
       a composition. Also called half tint.
       [1913 Webster]