1) A device designed to detect the presence of something and provide a signal in response. 2) A device designed to convert a range of incident energy of radiation into another form to determine the presence and/or amount of radiation. The device may function by electrical, photographic, or visual means.
1447, from L. detectus, pp. of detegere "uncover, disclose," from de- "un-, off" + tegere "to cover."