Meaning of great-circle sailing

great'-cir'cle sail"ing

Pronunciation: (grāt'sûr'kul), [key]
— Navig. Navig.
  1. sailing between two points more or less according to an arc of a great circle, in practice almost always using a series of rhumb lines of different bearings to approximate the arc, whose own bearing changes constantly unless it coincides with a meridian or the equator.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.