A porthole, sometimes called bull´s-eye window or bull´s-eye, is a generally circular window used on the hull of ships to admit light and air. Though the term is of maritime origin, it is also used to describe round windows on armored vehicles, aircraft, automobiles (the Ford Thunderbird a notable example) and even spacecraft.
On a ship, the function of a porthole, when open, is to permit light and fresh air to enter the dark and often damp below-deck quarters of the vessel. It also affords below-deck occupants a limited view to the outside world. When closed, the porthole provides a strong water-tight, weather-tight and sometimes light-tight barrier.
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