A small circular window in the side of a ship that can be opened or closed to let in light and air.
Compound of 'port' (the left side of a ship, from Old French 'port' meaning gate) and 'hole.' The term emerged in the 16th century as ships grew larger and needed ventilation.
Portholes are perfectly circular and waterproof by design—a porthole opening inward can flood a ship, so naval architects made them open outward to prevent disaster.
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