A proximity sensor is a component which is designed to detect the absence or presence of an object without the need for physical contact. They are non-contact devices, highly useful for working with delicate or unstable objects which could be damaged by contact with other types of sensor.
This non-contact operation also means that most types of proximity sensor (excluding types such as magnetic proximity sensors) have a prolonged lifespan. This is because they have semiconductor outputs, meaning that no contacts are used for output.
Proximity sensors are designed to provide a high-speed response (the interval between the point when the object triggers the sensor and the point when the output activates). Different types use varying sensing technologies but they all have the same purpose.