Parting strip

parting strip
A wood or metal strip fastened to the inside perimeter of a window frame used to separate adjacent sliding sash.
partition ...

Outside Stop - A strip of wood or metal fastened to the inside perimeter of a window frame that holds the sash against the parting strip.
Oriel - A box-like window projecting from the wall of a house.

See also: Window frame, Frame, Sash, Portico, Orientation