Transducer-type device that responds to a specific property of its surroundings or to a coming stimulus by a visually readable change in its state or by generation of a signal that can be read by an observer or instrument.
Examples: Photodiode, light-emitting diode, pH sensor, air-flow sensor, etc.
Source:
PAC, 2022, 94, 15. 'Glossary of terms relating to electronic, photonic and magnetic properties of polymers (IUPAC Recommendations 2021)' on page 47 (https://doi.org/10.1515/pac-2020-0501)