An electromagnetic pulse, or EMP, is burst of electromagnetic radiation from an explosion or a suddenly fluctuating magnetic field. It causes shorts in electrical devices. It has been appeared several times throughout the series.
In Exodus, Clark Kent placed the kryptonite key into his ship's slot to destroy the ship in order to stop Jor-El. The reaction from the ship caused a massive explosion, creating an EMP wave that overturned the truck with Jonathan and Martha Kent inside it. It also disrupted electrical power throughout the county, briefly delaying Lex Luthor's honeymoon flight.
In Reunion, Oliver Queen created an EMP arrow which could generate a small pulse that shorted out electrical devices nearby. Later, it was used by Duncan Allenmeyer in his astral form when he attempted to kill Oliver and Lex. However, they were rescued by Clark, who sped in front of the EMP arrow and reflected the pulse back to the source. Duncan was killed by the resulting energy surge.
In Metallo, Dr. Emil Hamilton gave Clark an EMP grenade to stop John Corben, who had been transformed into a cyborg with a kryptonite heart. After Clark found Corben, he used the grenade and was able to momentarily disable the cyborg, but Corben was able to restart almost instantly.