rhumb line

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The path of a ship that maintains a fixed compass direction, shown on a map as a line crossing all meridians at the same angle.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A line that cuts all meridians at the same angle.
• noun nautical The path of a vessel that maintains a constant compass direction.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a line on a sphere that cuts all meridians at the same angle; the path taken by a ship or plane that maintains a constant compass direction