1. A tool or a device that responds to a physical stimulus and produces an electronic signal: Sensors are increasingly being combined with RFID tags (Radio Frequency Identification tags) to detect the presence of a stimulus at an identifiable commercial location by using radio waves.

Some libraries use sensors at their exits in order to make sure that no one is taking materials away without making proper check outs.

2. Any detection instrument that is used to determine temperature, moisture, radiation, light, distance, or motion: There are all kinds of sensors which are being utilized to gain information that is essential for the well-being of humanity.

A device that detects the presence of an object and sends a signal to another device: Proximity sensors are often used on manufacturing lines to alert robots, or are routing devices on a conveyor belt to forewarn the presence of an object that is approaching.