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: Webster's Revised Unabridged Dictionary (1913)