EMP stands for ElectroMagnetic Pulse, a device capable of rendering technology in it's field of range useless or completely destroyed beyond repair. In the final chapter of Reconstruction (Chapter 19), Agent Washington activates the EMP to terminate the AI and other technology at command. The compound is seen to shut down, vehicles are disabled and Washington and the Meta are seen to be 'swallowed up' by the EMP, though they are later revealed to have survived. the fate of the AIs are unconfirmed. Sarge later tried to utilize the same energy from the EMP to build a turret for the warthog that had the unfortunate side effect of stalling the warthog whenever it was fired.
"Emp"
While EMP is pronounced as individual letters by Washington (as it should normally be), everyone he meets, even the computer terminal activating it, calls it an 'Emp', much to Washington's frustration.