The Collaborative International Dictionary of English v.0.48:

Easel \Ea"sel\, n. [D. ezel ass, donkey, hence, easel, or G.
   esel; akin to E. ass. See Ass.]
   A frame (commonly) of wood serving to hold a canvas upright,
   or nearly upright, for the painter's convenience or for
   exhibition.
   [1913 Webster]

   Easel picture, Easel piece, a painting of moderate size
      such as is made while resting on an easel, as
      distinguished from a painting on a wall or ceiling.
      [1913 Webster]

WordNet (r) 3.0 (2006):

easel
    n 1: an upright tripod for displaying something (usually an
         artist's canvas)