Ground

Ground is the name given to a reference voltage level in most electrical circuits, so-called since it is usual to connect it to an earth, which is a line connected (eventually) to an actual conductive rod driven into the ground.

Since it is usual to connect the metal chassis, etc to ground, this means that a person in contact with anything outside will not experience a shock when they touch any part of the device which is 'grounded' (i.e. connected to ground).