Electrostatic Discharge (ESD) is the large amount of current flow between any two points when a large (usually momentarily) potential difference is applied across those two points. If a large potential is applied on the gate of the MOS device, then a large current tends to flow through the gate and this in turn may disrupt the Silicon dioxide of the transistor. The silicon dioxide controls the important parameters like the threshold voltage (Vt) of the transistor and any physical damage would render the functionality of the entire device capricious.
Electro-static discharge is a single time event and is responsible for incurring a loss of millions of dollar to the semiconductor companies.
