The Collaborative International Dictionary of English v.0.48:

Casement \Case"ment\, n. [Shortened fr. encasement. See Incase
   1st Case, and cf. Incasement.] (Arch.)
   A window sash opening on hinges affixed to the upright side
   of the frame into which it is fitted. (Poetically) A window.
   [1913 Webster]

         A casement of the great chamber window.  --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

casement
    n 1: a window sash that is hinged (usually on one side)