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 smoke detector responded instantly by emitting a loud alarm when it detected the rising temperature."
rectifier that extracts modulation from a radio carrier wave
"The technician adjusted the detector to ensure it correctly extracted the audio modulation from the incoming radio carrier wave."
electronic equipment that detects the presence of radio signals or radioactivity
"The airport security officer adjusted his detector to scan for hidden metal objects and radioactive materials in the passenger's bag."
A device capable of registering a specific substance or physical phenomenon, and that optionally sounds an alarm or triggers a warning.
"The smoke detector registered the rising carbon monoxide levels and immediately began sounding its loud alarm."
An indicator showing the depth of the water in a boiler.
"The safety detector on the industrial boiler flashed red when the water level dropped too low."
In plain English: A detector is an instrument that finds something hidden by sensing it and giving you a signal.
"The smoke detector beeped loudly when we forgot to turn off the stove."
Usage: A detector is any instrument designed to sense a particular substance or signal, such as smoke detectors identifying fire hazards. While it often includes an alert mechanism like a siren, its primary function remains simply registering the presence of what it monitors.
The word detector comes from the Latin term dētēctor, which combines the root meaning to discover with a suffix that forms agent nouns. It entered English as a direct borrowing to describe someone or something that reveals hidden things.