Device which transmits an output signal in response to a physical input stimulus as voltage. In Earth observation a distinction between passives sensors, such as radiometers, and active sensors, such as radars, is common. Earth observation sensors – or instruments – are operated from different ground-/water-based, airborne, or spaceborne platforms.
Definition from CEOS-WGISS Glossary of Acronyms and Terms, v1.0
https://earthdata.nasa.gov/user-resources/glossary#visualization