magnetic monopole

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

• noun A hypothetical single pole of magnetism, as in a particle or a magnet with only a north or south pole, having net magnetic charge.
• noun An object containing a monopole.

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

• (Physics) a hypothetical subatomic particle having only one type of magnetic charge, the magnetic analogue of an electrically charged particle; it has only one magnetic pole instead of the two observed in all common magnetic object; -- it was still not observed as of 1998.

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

• noun physics a hypothetical particle having an isolated north or south pole; the magnetic equivalent of an isolated electric charge

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

• noun a hypothetical particle with a single magnetic pole instead of the usual two