An energy flux is the rate of transfer of energy through a particular object or surface.
In 2285, Commander Pavel Chekov picked up a minor energy flux reading on one of the USS Reliant's dynoscanners while scanning Ceti Alpha V. (Star Trek II: The Wrath of Khan)
In 2369, Constable Odo and Lieutenant George Primmin detected an energy flux on a Wadi ship docked with Deep Space 9 while running a scan on it trying to locate Commander Benjamin Sisko and three missing officers. (DS9: "Move Along Home")