1. Portion of a vertical plane beneath a segmental or semicircular vault running into it, bounded by the intrados and springing-line.
2. Similar-shaped aperture bounded by an arch or vault, e.g. in a wall at the end of a barrel-vault or above a door set in an arched opening, possibly a fanlight.
3. Tympanum in a segmental or semicircular pediment.
4. In a fortification a detached bastion, usually shaped like a half-moon.
5. Semicircular face of a Romanesque cushion-capital formed by the shaping of its lower part to fit the circular shaft.
lu·nette / loōˈnet/ • n. something crescent-shaped, in particular: ∎ an arched aperture or window, esp. one in a domed ceiling. ∎ a crescent-shaped or semicircular alcove containing something such as a painting or statue. ∎ a fortification with two faces forming a projecting angle, and two flanks. ∎ Christian Church a holder for the consecrated host in a monstrance.