A porthole is a small, usually round window in the side of a ship or boat. If you've ever been on a cruise ship or seen pictures of old sailing vessels, those circular windows looking out at the ocean are portholes. They're built round and sturdy because that shape helps them withstand the pressure of waves crashing against the hull.

Portholes serve a practical purpose: they let light and fresh air into cabins and lower decks while keeping water out. On submarines, portholes need to be especially strong since they have to hold back the enormous pressure of deep water. Many portholes can be sealed shut with heavy metal covers called deadlights when the weather turns rough or the seas get dangerous.

Despite the name, portholes appear on both sides of a ship. Modern ships and airplanes still use portholes, though airplane windows are usually oval rather than perfectly round. When you look through a porthole on a ship, you're seeing the world the same way sailors have for hundreds of years: framed by a small circle of glass between you and the vast ocean beyond.