Ah, the rum route. Echoes of the triangle trade...
I thought it referred to the navigational term, "rhumb line". The rhumb line is a course of constant compass heading from one point on the globe to another, it corresponds to a straight line between two points drawn on a Mercator Projection.
A Great Circle course is a geodesic curve which is the shortest distance between two points on a sphere. A GC involves constantly changing compass heading, and corresponds to a straight line drawn on a Gnomonic projection.
This is why when Lindbergh flew from New York to Paris he passed near Newfoundland and Iceland.