An "on/off sensor" typically refers to a device that can detect whether a specific condition is present or not and then switch between two states accordingly, turning something on or off.

For example, a motion sensor in a room can detect whether someone is present and turn the lights on or off accordingly. In this case, the sensor is an on/off sensor because it switches between two states: on (lights are turned on) or off (lights are turned off).

Overall, an on/off sensor is a useful device for automating systems and making them more efficient by turning them on or off based on specific conditions