great circle

isoympyrä, isoympyräreitti puhekieltä A circle defined as the intersection of the surface of a sphere and a plane which passes through the centre of the sphere.
A segment of such circle representing the shortest distance between two points on the surface of a sphere.
1902, S. L. Penfield, "Use of the Stereographic Projection for Geographical Maps and Sailing Charts", in American Journal of Science

Wherever a navigator may be, it certainly would be to his advantage to be able to find the great circle, or shortest course, and the distance, to his port of destination, ....

The shortest distance between two points on an ocean Mercator chart is not a straight line - it is a curve known as a 'great circle'.

Because the Earth's surface is curved, the shortest distance is actually a curve over the surface called a great circle.