Nautical. Originally: an aperture in a ship's side through which a cannon may be pointed. Subsequently: a small window (usually circular) in the…

Military. An opening widening from within made in an epaulement or parapet for the purpose of allowing a gun to be fired through it.

a port-hole for a gun.

Nautical. An opening in the side of a ship for entrance and exit, or for the loading and unloading of cargo; (also) an aperture in the side of a…