magnetic equator

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

• noun A line connecting all points on the earth's surface at which a magnetic needle balances horizontally without dipping.

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

• noun An imaginary line paralleling the equator where a magnetic needle has no dip, the dipping needle being horizontal; called also aclinic line.

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

• noun military A line drawn on a map or chart connecting all points at which the magnetic inclination (dip) is zero for a specified epoch.

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

• noun an imaginary line paralleling the equator where a magnetic needle has no dip