magnetic compass

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

• noun An instrument that uses a magnetized steel bar to indicate direction relative to the earth's magnetic poles.

from the GNU version of the Collaborative International Dictionary of English.

• noun An device using a magnet (such as a magnetic needle) to indicate the direction of magnetic north.

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

• noun military (nautical) An instrument containing a freely suspended magnetic element which displays the direction of the horizontal component of the Earth’s magnetic field at the point of observation.

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

• noun compass based on an indicator (as a magnetic needle) that points to the magnetic north