Lunette

In architecture, a lunette (French lunette, "little moon") is a half-moon shaped space, either filled with recessed masonry or void. A lunette is formed when a horizontal cornice transects a round-headed arch at the level of the imposts, where the arch springs. If a door is set within a round-headed arch, the space within the arch above the door, masonry or glass, is a lunette. If the door is a major access, and the lunette above is massive and deeply set, it may be called a tympanum.