Rhumb Line
A Rhumb Line (or loxodrome) is a line on the surface of the earth that crosses every meridian at the same angle — that is, a track of constant compass course. It appears as a straight line on a Mercator chart, which is exactly why that chart projection is used for navigation: a course laid off as a straight line is a rhumb line the ship can steer without changing heading.
Over short and medium distances the rhumb line is the practical route. Over long ocean passages it is noticeably longer than the great circle, so long routes are sailed as a series of rhumb-line legs between waypoints that approximate the shorter great circle.
The rhumb line is the practical, steerable route — a single compass course — and the reason marine charts use the Mercator projection. Knowing when a rhumb line is good enough and when a great circle saves worthwhile distance is a basic passage-planning judgement.
Why is a rhumb line a straight line on a Mercator chart?
The Mercator projection is designed so that a line of constant compass course plots as a straight line, making it easy to lay off and steer courses. This is why it remains the standard navigation chart projection.