What does "lunette" mean?

Additional senses

1. 2.A kind of watch crystal which is more than ordinarily flattened in the center; also, a species of convexoconcave lens for spectacles.
2. 3.A piece of felt to cover the eye of a vicious horse.
3. 4.(Arch.) Any surface of semicircular or segmental form; especially, the piece of wall between the curves of a vault and its springing line.
4. 5.An iron shoe at the end of the stock of a gun carriage. Lunette window (Arch.), a window which fills or partly fills a lunette.