A window created with a single pane of glass, of which there may be more than one to fill a wall or a large opening.
Picture windows are usually large, large enough to encompass the view beyond, smaller versions tend to be called ‘single pane’ windows. Picture windows may be fixed or able to open; opening picture windows either slide or fold back, or very smart ones disappear neatly into the ground beneath.
They can be wall-to-wall, floor to ceiling but as long as it is large enough to make the outlook feel like a framed picture, it’s a picture window.