Other forms: portholes
A porthole is a small round window in the side of a ship. Look through the porthole, a sealed porthole, a cabin with a porthole.
It is round because a circle has no corners for stress to concentrate at, which is the same reason aircraft windows are rounded — square ones cracked and brought aircraft down in the 1950s. The heavy glass and the metal cover make it watertight.
1nounportholeA small round window in the side of a ship.
“Through the porthole of our cabin, we could see the grey waves.”
porthole is a window.
Port plus hole. Port here means the side opening of a ship, from Latin porta, a gate.
From port + hole.
Etymology from Wiktionary, CC BY-SA