A porthole is the small round window in the side of a ship, and you also see them on some planes. It is round and thick because the water, or the air outside a plane, pushes hard against it; a square window would break more easily. On a ship you can often close it tight with a metal cover when the waves are high. People sometimes joke that any tiny round window looks like a porthole, even on a house. It is a plain word for a real object — not fancy, not slang.