noun

• (architecture) A small opening in a vaulted roof of a circular or crescent shape.
  "Nanny Broome was looking up at the outer wall. Just under the ceiling there were three lunette windows, heavily barred and blacked out in the normal way by centuries of grime."
• (architecture) A crescent-shaped recess or void in the space above a window or door.
  "The decoration of a lunette discovered in the Duomo of Pistoia in the 1950s, which represents Christ blessing and Saints James and John, all bust-length, constitutes a more remarkable and stylistically more advanced approach to painting."
• (obsolete) An image or other representation of a crescent moon.
  "The lesser portions of the tablet has over this Mithras, a lunette or symbol of the moon, who, according to Porphyry's comment, is the queen of generation and as such was denominated by the ancients both a bee and a bull [...]."
• (fortifications) A field work consisting of two projecting faces forming a wedge each of which extends from one of two parallel flanks.
  "This Lunette, as we have seen, was confronted, and even in siege-form "approached", by a part of Canrobert's army [...]."