(Terms of radiometry)
Radiant flux is radiant energy per unit time.
As all radiation related terms, this is not very intuitive to understand. If we use an analogy of flowing water, then the energy relates to the absolute amount of water travelling a certain vertical distance, while the flux or power is the fraction of this amount travelling the same distance in a given time.
Standard unit for radiant flux is Watt (W),
which is defined as Joules per Second (J/sec).
References: 
radiant energy radiometry 

 
English  German  
radiant flux  Der Strahlungsfluß 