What does the term 'lubber line' refer to?

The term 'lubber line' refers specifically to a compass direction. In navigation, the lubber line is a straight line on a compass that indicates the direction the vessel or diver is currently facing. It is essential for determining bearing and navigation as it aligns with the direction of travel. This helps divers and navigators understand the cardinal directions and effectively plan their underwater routes or surface navigation.

A, C, and D relate to aspects of underwater activities but do not define what a lubber line is. Depth measurement typically refers to the vertical distance below the surface, while underwater navigation encompasses various techniques used to determine one's position and course underwater. A dive plan outline involves the organization of a dive, including objectives and safety measures, rather than compass reading.