detector

Short definition from Wiktionary

Noun

1. 1
   A device capable of registering a specific substance or physical phenomenon, and that optionally sounds an alarm or triggers a warning.
2. 2
   A device capable of registering a specific substance or physical phenomenon, and that optionally sounds an alarm or triggers a warning: An indicator showing the depth of the water in a boiler.
3. 3
   A device capable of registering a specific substance or physical phenomenon, and that optionally sounds an alarm or triggers a warning: A galvanometer, usually portable, for indicating the direction of a current.