magnetic needle

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

• noun A slender bar of magnetized steel usually suspended on a low-friction mounting and used in various instruments, especially in the magnetic compass, to indicate the direction of the earth's magnetic poles.

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

• noun A slender magnet suspended in a magnetic compass on a low-friction mounting; used to indicate the direction of the earth's magnetic pole.

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

• noun a slender magnet suspended in a magnetic compass on a mounting with little friction; used to indicate the direction of the earth's magnetic pole