What term means downstairs on a ship?

The term that means downstairs on a ship is 'below.' In nautical terminology, 'below' typically refers to any location situated beneath the main deck of the vessel. This is important for understanding ship layout and navigation, as it helps to establish orientation when discussing different parts of the ship.

The other terms have distinct meanings in maritime language. 'Upper deck' refers to the higher levels of a ship, above the main deck, and does not relate to areas that are below it. 'Fore' indicates the front part of the ship, while 'after' pertains to the rear. Neither of these directional terms addresses the concept of being below deck, which is essential for comprehending the layout and operation of a vessel.