A sash window is a vertically sliding window, usually with two panes which overlap when the window is open. It is most closely related to a sliding window, which opens by sliding horizontally. Sash windows are especially common in buildings from the early 20th century.

Vertical movement distinguishes this type from a sliding window, which moves sideways. The date of the building is not a condition of the sash type.

Two overlapping panes are usual but are not mandatory. The defining action is vertical sliding, without swinging the window around a hinge.