EMC translates to electromagnetic compatibility, i.e. the ability of electrical or electronic devices to function properly in their intended electromagnetic environment, and without introducing unwanted electromagnetic disturbance in that environment.
EMC is a measure of quality. It tells how well a device can cooperate with other devices, electrically. There are, of course, a number of international standards dealing with and defining the various aspects of EMC.
Electromagnetic energy is transmitted by electrically conductive materials, or by electromagnetic waves in space (“in the air”). Conductive materials are usually metals in cables, cable screens and enclosures. Many liquids are conductive. Water is more or less conductive, unless it has been distilled. Salt water and rain water in coastal, industrial or urban areas are conductive. Electromagnetic disturbance is any electromagnetic phenomenon, which may degrade the performance of a device, equipment or system, or adversely affect living or inert matter.