A reference voltage level of "zero potential" for electric and electronic circuits. For most equipment plugged into an AC outlet, the word ground generally refers to the earth, and the metal parts are grounded ("chassis ground") to the earth for safety. In the case of a short circuit, the current flows through the green wire and third prong in the power cord (U.S. and Canada), which causes the circuit breaker to trip.
Power supplies, circuit boards and signal pathways in most electronic equipment are also connected to the same earth ground as the chassis for reference voltage and safety. There are exceptions; for example, medical equipment that is attached to a patient is generally isolated from earth ground even when plugged in, because the time between a short circuit and the breaker tripping could prove fatal. Ethernet adapters and other networking equipment are also isolated from earth ground in order to prevent unbalanced ground loops from causing interference (see ground loop
Every portable, battery-operated device, such as a laptop and iPod, as well as the electrical systems in every vehicle, have their own zero reference voltage. Naturally, these systems are not connected to earth ground, but the reference voltage is still called ground.
At the electric panel in U.S. and Canadian buildings, the ground lines are wired to the neutral line of the electric service, to the earth via a ground rod and to the metal plumbing pipes. In large datacenters, a separate ground system may connect every server via heavy-duty cable to the metal structure of the building to ensure that the voltage reference between all equipment is the same. See ground loop