An Electro-Magnetic Pulse (EMP) is a sudden, massive broad-band wave of electro-magnetic energy that strikes large regions of the earth.   It can originate from the sun in the form of solar events or as a result of a detonated nuclear device.  In either case, an EMP attack can severely damage electronic devices resulting in the disabling of computers, the electric grid, telephones, water supply and transportation systems of an entire continent.