The Collaborative International Dictionary of English v.0.48:

Impressionism \Im*pres"sion*ism\, n. [F. impressionnisme.] (Fine
   Arts)
   The theory or method of suggesting an effect or impression
   without elaboration of the details; -- a disignation of a
   recent fashion in painting and etching.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Impressionism
    n 1: a school of late 19th century French painters who pictured
         appearances by strokes of unmixed colors to give the
         impression of reflected light