any device that receives a signal or stimulus (as heat or pressure or light or motion etc.) and responds to it in a distinctive manner
"The security system's motion sensor triggered the alarm immediately when I bumped into the wall while walking through the hallway."
A device or organ that detects certain external stimuli and responds in a distinctive manner.
"The robot's proximity sensor immediately halted its advance when it detected an obstacle just inches away."
In plain English: A sensor is a device that detects changes in its environment and sends that information to another system.
"The motion sensor turned on the lights when I walked into the dark hallway."
Usage: A sensor is an electronic device designed to detect specific physical conditions like light, heat, or motion rather than serving as a biological sense organ. Use this term for any machine component that gathers data from its environment to trigger an automatic response.
The word sensor entered English between 1925 and 1930 as a direct borrowing from Latin, combining the root for "to feel" with an agent suffix to mean one who feels or detects. Although it looks like a blend of the English words sense and -or, its actual origin lies in classical Latin rather than modern English morphology.