• The hard stuff you hit when you fall out of an airplane.
• In electricity, a large conducting body, such as the earth or an electric circuit connected to the earth, used as a reference zero of electrical potential.
• A conducting object, such as a wire, that is connected to such a position of zero potential for the sake of "grounding" an electronic device.
• Grounding - What a quarterback does with a football when he sees Reggie White coming at him.
• Verb - to "ground" something means connecting it electrically to ground.

In electricity an electrical path to ground is provided by code at all electrical outlets for safety. In the event of a failure or circumstance that might normally cause the operator (or anyone coming in to electrical contact with connected equipment) to be the return path for the electrical current the ground can shunt this current away safely and return it to earth, which is where it is trying to go anyway. The alternative of the operator being the return path can cause electrocution.