[250] If you answered "grounding," you got the wrong answer. Article 250 provides the requirements for grounding and bonding. That's an important distinction, because many people misapply grounding in place of bonding. This creates enormously expensive problems as well an dangers to people.
 

[250.2] It's an intentionally constructed, permanent, low-impedance path for current under ground fault conditions.
 

[250.2] A ground fault is an unintentional connection between an ungrounded conductor and bonded metallic objects (which the NEC refers to as "grounded," but see the Article 100 definitions of "grounded" and "bonded").
 

[250.2] It's the path electricity takes in the event of a ground fault. What you want is an intentional ground fault path that directs ground fault current properly. Otherwise, the ground fault path may include people and equipment.
 

[250.2 FPN] Examples include piping, metal frameworks, cable sheaths, ductwork, and other conducting bodies.