A Poynting vector indicates the directional energy flux-density (energy transfer in watts per square meter) of an electromagnetic field. It was conceived by John Henry Poynting, who worked out that it is related to the electric and magnetic fields as follows:
S = E × H
Electromagnetic radiation is a pattern in the EM field that propagates as waves, and for it, the Poynting vector matches the energy carried by the waves. The net flow of energy due to waves and other changes in the field is termed the Poynting flux, the Poynting flux vector showing its magnitude and direction. When speaking of a single ray of light, or a single photon, the Poynting flux associated with it would be the same as its Poynting vector, thus in some contexts, either term might be used.
A Poynting flux jet is a jet (e.g., from an active galactic nucleus) for which the majority of the energy is through the EM and EMR, i.e., the type of energy EM field for which Poynting vectors apply. Such jets often show relativistic speed. Mass loading can make the jet change over its distance to a more conventional jet.