What does the term 'active galactic nucleus' specifically refer to?

The term 'active galactic nucleus' (AGN) specifically refers to a region at the center of a galaxy that emits an unusual amount of energy, often outshining the combined light of all the stars in the galaxy. This energetic activity is primarily due to the presence of a supermassive black hole surrounded by an accretion disk of gas and dust. As material spirals into the black hole, it heats up and emits radiation across the electromagnetic spectrum, including radio, optical, X-ray, and sometimes gamma-ray wavelengths.

Active galactic nuclei can take different forms, such as Seyfert galaxies, quasars, and blazars, depending on their specific characteristics and orientation. The defining feature that unites all AGNs is their extraordinary luminosity and the diverse physical processes that occur near the central black hole, marking them as significant objects of study in astrophysics and cosmology.