Oriel window in the context of Latticed light

A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent material, a sash set in a frame in the opening; the sash and frame are also referred to as a window. Many glazed windows may be opened, to allow ventilation, or closed to exclude inclement weather. Windows may have a latch or similar mechanism to lock the window shut or to hold it open by various amounts.

Types include the eyebrow window, fixed windows, hexagonal windows, single-hung, and double-hung sash windows, horizontal sliding sash windows, casement windows, awning windows, hopper windows, tilt, and slide windows (often door-sized), tilt and turn windows, transom windows, sidelight windows, jalousie or louvered windows, clerestory windows, lancet windows, skylights, roof windows, roof lanterns, bay windows, oriel windows, thermal, or Diocletian, windows, picture windows, rose windows, emergency exit windows, stained glass windows, French windows, panel windows, double/triple-paned windows, and witch windows.

A bay window is a window space projecting outward from the main walls of a building and forming a bay in a room. A bow window is a form of bay with a curve rather than angular facets; an oriel window is a bay window that does not touch the ground.

A window may be all three: projecting outward from the main fascia of a wall, curved in shape, and not reaching the ground. A bay window may be supported from the ground by a foundation, or in space by corbels, brackets, or cantilever.

A Chicago window is a large fixed glass panel flanked by two narrower sashes of the same height, filling a structural bay. The large pane is a single panel of plate glass, and the flanking elements are vertical double-hung sash windows with no dividing muntins. The fenestration was first used by architect William LeBaron Jenney in the 1884 Home Insurance Building, and immediately after by several of the Chicago School architecture firms such as Holabird and Root in their Marquette Building (Chicago), Daniel Burnham's Fisher Building (Chicago), and Louis Sullivan's Carson Pirie Scott department store. The window design was made possible by advances in glass-making technology and steel structural framing, and became a defining feature of the Chicago school style. The design offered both abundant natural light and practical ventilation. Projecting oriel bays are a common variant of the Chicago window, as seen here in the Reliance Building (1895) by Burnham and Root.