Relativistic Beaming (or Doppler Beaming or
Doppler Boosting or Headlight Effect)
is a factor in the apparent Luminosity of an object
when it is moving at Relativistic Speed relative to the observer,
due to an exaggerated Doppler effect.
When an object is moving at relativistic speeds toward or away
from you, its Electromagnetic Radiation, in turn, appears brighter or dimmer
than would be seen by an observer stationary relative to the same object.
This effects relativistic Jets, e.g., from Quasars
and Active Galactic Nuclei, making them appear brighter or dimmer
according to the direction of the jets.