A window that opens on hinges at the side, like a door.
From Old French 'cassement' meaning 'frame' or 'border,' derived from 'casse' meaning 'case' or 'box.' The window got its name because it sits within a 'case' or frame that allows it to swing open. The root is the same as 'case' — casement windows are literally 'cased' windows, sitting in frames that act like boxes or cases. The hinged opening mechanism was secondary to the framing concept in the original naming.
Casement windows are named after boxes! The 'case' part comes from the frame that surrounds and contains the window, like a box holds its contents. Before casement windows, most windows were just holes in walls with shutters — the revolutionary idea was creating a 'case' or frame that could hold glass and still allow the window to open.
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