Radio slave
From The Glossary of Digital Photography
A device attached to a camera that is used to trigger electronic flashes by radio signal. This is very useful for photographers to control a number of electronic flashes at a distance from the camera without stringing wires from one position to another. For example, a wedding photographer can set up several flashes at a wedding reception to capture a couple dancing in relative darkness. By using radio slaves rather than optical slaves, the photographers flash units are not triggered when guests take flash photographs.
