Source : Webster's Revised Unabridged Dictionary (1913)

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.

         A casement of the great chamber window.  --Shak.

Source : WordNet®

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