A momentary switch is a switch that only turns on during an interaction and that automatically turns off when the interaction ends. Switches are electrical components that control whether a circuit is open or closed.
Momentary switches only complete a circuit when they are interacted with in some way. They stay on for the duration of the interaction, but once it ends, they put the circuit back into an off state. One commonly encountered kind of momentary switch is the keyboard button. When a button on a keyboard is pressed, it completes a circuit and signals the computer to do something, such as type a letter or move a cursor.
Switches are used throughout electrical engineering to put devices into certain states. When a circuit is closed, electrical current can flow freely from its starting point, travel along a path and return to the start with no impediments. A closed circuit is in an "on" state. When a circuit is open, current cannot flow back to its starting point, and the circuit is in an "off" state.