1) A frame of a window or part of a window, opening on hinges attached to the upright side of the frame (OED).
1565 j stancyon of iron and a barre, vj casements of iron for windowes, Knaresborough
1612 1 East glasse window with 2 iron casements & sneckes finely wrought, Brafferton
1676-7 his son and his hostler are frequently out of his howse at unseasonable tymes of the night and coms in over the topp of the howse and creeps in at a casement when the doores are locked, Halifax. Note: 1394-5 Item Carpentario ... pro factura unius cas ad fenestram refectorii iiijd, Whitby.