What term is used for any portion of the main deck that is outside?

The term "weatherdeck" refers specifically to any part of the main deck of a ship that is exposed to the elements, meaning it is outside and accessible to weather conditions. This is crucial in naval operations as it affects how the ship navigates and responds to the ocean environment. Weatherdecks often serve vital functions for operations conducted at sea, such as launching and recovering small boats, and hosting personnel while they are conducting various duties.

In contrast, freeboard refers to the distance from the waterline to the deck of the ship, measuring how high the ship sits in the water and providing insight into its buoyancy and stability. Lifelines are safety features on a ship, consisting of ropes or cables that provide a barrier to prevent personnel from falling overboard; they are not a structural part of the deck. Lastly, the term stem refers to the forwardmost part of the ship, relating to the ship's bow. Thus, the weatherdeck distinctly identifies portions of the main deck that are outside, while the other terms focus on different aspects of naval architecture and safety.