Terminology 

beamed, beaming 

shorter terms for ‘relativistic beaming’

beta 

the ratio of the jet speed to the speed of light, sometimes called ‘relativistic beta’

core 

region of a galaxy around the central black hole

counter-jet 

the jet on the far side of a source oriented close to the line of sight, can be very faint and difficult to observe

Doppler factor 

a mathematical expression which measures the strength (or weakness) of relativistic effects in AGN, including beaming, based on the jet speed and its angle to the line of sight with Earth

flat spectrum 

a term for a non-thermal spectrum that emits a great deal of energy at the higher frequencies when compared to the lower frequencies

intrinsic luminosity 

the luminosity from the jet in the rest frame of the jet

jet (often termed 'relativistic jet')

a high velocity (close to c) stream of plasma emanating from the polar direction of an AGN

observed luminosity 

the luminosity from the jet in the rest frame of Earth

spectral index 

of measure of how a non-thermal spectrum changes with frequency, the smaller α is the more significant is the energy at higher frequencies, typically α is in the range of 0 to 2

steep spectrum 

a term for a non-thermal spectrum that emits little energy at the higher frequencies when compared to the lower frequencies