Electromagnetic Interference 
Any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronics and electrical equipment. It can be induced intentionally, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like.
EMI is said to exist when unwanted voltages or currents are present so that they adversly affect the performance of an electrical device or electronic system. These voltages/currents can reach the victim circuit or device by conduction or by nonionizing radiation. In all cases, EMI occurs because of a combination of three factors:
- A source
- A transmission path
- A response (at least one response is unplanned)
Functional interference often includes: sine waves, computer clock pulses, speech or video waves, or pulses forming data trains. An example of this interference type is signal leakage from cable TV systems. Fluorescent lamps, commutators, car ignition systems, and industrial, scientific and medical equipment all constitute sources of interference - and this includes EMP that accompanies a nuclear explosion.
EMI always starts with current flow through a conductor and also shows up in the victim equipment in the form of a current or voltage. The coupling path (including a conducting gas or air) might be a conduction or radiation path. The actual paths can include common wiring, capacitance between devices, mutual inductance between adjacent wiring, nonionizing radiation, or wires in an EM field. This type of coupling is aided by the fact that all conductors exhibit resistance and inductance.