- an opening in a ship's side, esp. a round one for admitting light and air, fitted with thick glass and, often, a hinged metal cover
- an opening to shoot through, in the wall of a fort; embrasure
- an opening shaped somewhat like this, as in a furnace door
- Nautical A small, usually circular window in a ship's side.
- An opening in a fortified wall; an embrasure.