n.1 an arched aperture in a domed ceiling to admit light.
2 a crescent-shaped or semicircular space or alcove which contains a painting, statue, etc.
3 a watch-glass of flattened shape.
4 a ring through which a hook is placed to attach a vehicle to the vehicle towing it.
5 a temporary fortification with two faces forming a salient angle, and two flanks.
6 RC Ch. a holder for the consecrated host in a monstrance.
Etymology: F, dimin. of lune (see LUNE)