Remote sensor
From The Glossary of Digital Photography
A sensor that can be used to fire an electronic flash that is not attached to the camera by a wire. The sensor can use radio signals, infrared signals, or the visible light from another flash firing to operate the electronic flash attached to the sensor. Some flashes have a built-in remote sensor. These are two-part devices; they have a sender unit attached to the camera and a receiver unit attached to the flash so that the flash can be fired remotely.
