Which term describes the line of constant compass direction?

The term that describes the line of constant compass direction is the rhumb line. A rhumb line represents a path of constant bearing across the Earth's surface. It is important to note that while navigating, following a rhumb line means steering at the same compass heading, regardless of the curvature of the Earth.

This concept is particularly useful for navigation because it simplifies the process of planning and maintaining a course over long distances. On a Mercator projection, a common type of map used for navigation, a rhumb line appears as a straight line, making it easier for navigators to plot and follow.

In contrast, the other options refer to different navigation concepts. The course line typically refers to the intended direction intended to be followed, while the track line indicates the path that the vessel has actually followed over the water. Waypoint lines would not be a standard term used in navigation; rather, waypoints are specific GPS coordinates or locations that the navigator might use to assist in plotting a course.