In classical architecture a low-pitched triangular gable above a facade, or a smaller version over porticos above the doorway or above a window.

A triangular gable end of the roof above the horizontal cornice, often with sculpture.

Illustration from the Albright-Knox Art Gallery

Broken pediment: A pediment open or broken either at the apex.

Open-bed or broken-bed pediment: pediment with a gap in the base molding.

Information provided by: http://ah.bfn.org