From Halopedia, the Halo wiki
EMP is a medal awarded for stripping the enemy of its shields. It was featured in Halo 3 as the EMP Medal, in Halo 3: ODST as the EMP Kill, in Halo: Reach as the EMP Blast and in Halo 4 as the EMP Assist.
The medal can be awarded in certain modes in each Halo title. In Halo 3, the medal is only awarded in campaign when the player removes a Brute's or Jackal's shields with a Plasma pistol or a Power Drain, then quickly killing the opponent. Similar applies to Halo 3: ODST's campaign and Firefight mode when a player does so to a Brute, Jackal or an Engineer.
In Halo: Reach, the medal is awarded in campaign, Firefight and multiplayer for killing an enemy after collapsing its shields with an EMP burst from a grenade launcher, an overcharged plasma pistol bolt, or Armor lock. The player will also receive the medal if their opponent is finished off by another player.
In Halo 4, however, the medal is only awarded for removing an opponent's shields who is then killed by an ally.
In its first three titles, the medal is represented by a 16-pointed green star over a 16-pointed gold star, with an overcharged plasma pistol bolt over the green star. In Halo 4, it is represented by a three lightning bolts shooting out of a blue and white Yin and Yang on a blue circle and inverted grey pentagon.
Performing ten EMP kills against Brutes in any level of Halo 3: ODST unlocks the My Clothes! achievement.