Dormer window. A window placed vertically in a sloping roof and with a roof of its own. It usually serves as sleeping quarters, hence the name. Also called a lucarne. The gable above a dormer window is often formed as a pediment and called a dormer head.
-- John Fleming, Hugh Honour & Nikolaus Pevsner, 1991
The Penguin Dictionary of Architecture
Penguin Books, London, England