What is the general term used to describe empty spaces within a ship's structure?

The general term used to describe empty spaces within a ship's structure is "voids." Voids are intentionally designed spaces that can serve multiple functions, including contributing to the overall buoyancy of the vessel and allowing for the safe passage of water in case of flooding in specific compartments. They play a crucial role in enhancing the ship's structural integrity and stability while also allowing for weight distribution adjustments as needed.

Compartments are more specifically defined areas within a ship that serve distinct functions, such as sleeping quarters or engine rooms, and typically contain equipment or storage. Ballasts pertain to specific weights added to a ship to ensure stability and balance but do not on their own refer to empty spaces. Buoys are floating devices used typically to mark navigational hazards or mooring points in water and are not part of a ship's internal structure. Therefore, the term "voids" accurately encapsulates the concept of empty spaces within a ship, making it the correct choice.