lunette.
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.

lunette (clay dune) An accumulation of aeolian sediment, consisting of clay pellets the size of sand grains, found around the margins of some salt lakes.

lunette XVI. — F. lunette, dim. of lune moon; see -ETTE.