NO/NC

Normally Open and Normally Closed

These terms refer to the contacts on a switch or relay. The NO contact is normally open (breaking the circuit) in the default state. The contact gets closed when the button is pressed, switch is turned on, or relay is energized. The NC contact is the opposite.

The two types of contacts can be paired so a common pin is normally connected to the NC contact but instead gets connected to the NO contact upon activation.