A blazar is a class of active galaxy that includes BL Lacertae objects, high polarization quasars (HPQs), and optically violent variables (OVVs). Blazars are characterized by strong, compact, flat-spectrum radio emission. They also show continuum domination of the optical emission, and/or significant optical polarization, and/or strong gamma-ray emission, and/or significant changes in optical flux on short time scales. They are believed to be active galactic nuclei (AGM) whose jets are aligned within 10° of our line of sight.

In the early 1990s, the Compton Gamma Ray Observatory was used to discover a new class of blazar known as a gamma-ray blazar, whose members appear to have gamma-ray-emitting jets that are more tightly bound than the usual radio jets. These gamma ray-loud blazars may also be a major source of the cosmic diffuse background above energies of about 100 MeV.