A blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet composed of ionized matter traveling at nearly the speed of light) directed towards an observer.

Blazars are powerful sources of emission across the electromagnetic spectrum and are observed to be sources of high-energy gamma ray photons.

Blazars are highly variable sources, often undergoing rapid and dramatic fluctuations in brightness on short timescales (hours to days).

In 2009, a team of astronomers using the Swift spacecraft used the luminosity of S5 0014+81 to measure the mass of its super-massive black hole. They found it to be about 10,000 times more massive than the black hole at the center of our galaxy, or equivalent to 40 billion solar masses