A "witch window"

In American vernacular architecture, a witch window (also known as a Vermont window, a coffin window, or a sideways window) is a window (usually a double-hung sash window, occasionally a single-sided casement window) placed in the gable-end wall of a house and rotated approximately 1/8 of a turn (45 degrees) from the vertical, leaving it diagonal, with its long edge parallel to the roof slope. This technique allows a builder to fit a full-sized window into the long, narrow wall space between two adjacent roof lines. These windows are found almost exclusively in or near the U.S. state of Vermont, principally in farmhouses from the 19th century...

Photo and text from Wikipedia, where there is an explanation of the reasons for such windows, and the curious names applied to them.