This article is from the Astronomy FAQ, by Joseph Lazio (jlazio@patriot.net) with numerous contributions by others.
Short answer is that magnetic monopoles are the magnetic equivalent of
point electric charges. Like the electron and positron (which can be
considered to carry one unit of electric charge, negative and
positive, respectively), one could imagine that there might be
magnetic particles which have only a north or south magnetic pole.
See J. D. Jackson, _Classical Electrodynamics_, for an extensive
discussion.
 
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