In electrical engineering, NO and NC stand for "Normally Open" and "Normally Closed" respectively, and they refer to the states of a switch or a relay contact when they are in their resting or unactuated position.

• Normally Open (NO): In this state, the switch contacts or relay contacts are open (i.e., not conducting electricity) when there is no external force applied to the switch or relay coil. When the switch is actuated or the relay coil is energized, the contacts close, allowing current to flow through them.

• Normally Closed (NC): In this state, the switch contacts or relay contacts are closed (i.e., conducting electricity) when there is no external force applied to the switch or relay coil. When the switch is actuated or the relay coil is energized, the contacts open, interrupting the current flow.

These terms are commonly used in various electrical and electronic applications, such as control circuits, automation systems, and safety mechanisms. The designation of NO or NC indicates the behavior of the switch or relay contacts in their default or unactuated state.