Pediment. Not a Greek or Roman term but signifying in classical architecture a low-pitched gable above a portico, formed by running the top member of the entablature along the sides of the gable; also a similar feature above doors, windows, etc. It may be straight sided or curved segmentally.
-- John Fleming, Hugh Honour & Nikolaus Pevsner, 1991
The Penguin Dictionary of Architecture
Penguin Books, London, England