1) Physical quantity 
Energy flux is the rate of transfer of energy through a surface. The quantity is defined in two different ways, depending on the context: ...
quotient of dR by dt, where dR is the increment of radiant energy in the time interval of duration dt: = dR/dt NOTE - This quantity is expressed in watts (W).
The rate of energy flow from, into, or through a substance.
The rate of flow of energy through a reference surface. In cgs units, measured in erg s-1. Also measured in watts, where 1 watt = 1 x 107 erg s-1. Flux density, the flux measured per unit area, is also often referred to as 'flux'.