A ground is a conducting connection between an electrical circuit or equipment and the earth, or between an electrical circuit and some conducting body that serves in place of the earth.
The purpose of a ground is to prevent the buildup of voltages that may result in a hazardous situation for the connected equipment and for the person operating the equipment. A 3-prong plug, as shown in the image on the right, has a grounding wire connected to the third prong (indicated by the arrow). When plugged into a 3-prong wall receptacle the grounding wire connects from the electrical circuit to the equipment.
All laboratory instruments and appliances must be adequately grounded.