What is the name given to a small circular window in the side of a ship?

The correct term is 'scuttle' and the hinged cover a 'deadlight'

http://www.nor-mek.com/window/side.html

scut·tle
n.
-A small opening or hatch with a movable lid in the deck or hull of a ship or in the roof, wall, or floor of a building.
-The lid or hatch of such an opening.

dead·light
n.
Nautical.
-A strong shutter or plate fastened over a ship's porthole or cabin window in stormy weather.
-A thick window set in a ship's side or deck.
-A skylight constructed so that it cannot be opened.

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