sensor

What it means

1. A device or organ that detects certain external stimuli and responds in a distinctive manner.

   The smoke sensor triggered the alarm.

   2 more examples

   Modern phones contain a light sensor that adjusts brightness.

   A motion sensor detected movement in the empty room.

In a sentence

The smoke sensor triggered the alarm.

Where it comes from

Originated 1925–30, formed as a Latin loanword from sentiō (“to feel”) + -tor (“-er”). By surface analysis, sense + -or.