A sash window is made of one or more movable panels or "sashes" that form a frame to hold panes of glass which are often separated from other panes (or "lights") by narrow bars.  Although any window with this style of glazing is technically "a sash", the term is used almost exclusively to refer to windows where the glazed panels are opened by sliding vertically. The design of the sash window is attributed to the English scientist and inventor, Robert Hooke and the oldest known examples of sash windows were installed in England in the 1670s, for example at Ham House, London.

Sash windows are often found in Georgian and Victorian houses, and the classic arrangement has three panes across by two up on each of two sashes, giving a "six over six" panel window, although this is by no means a fixed rule.