Final answer:
An active galactic nucleus refers to the highly energetic central region of certain galaxies. These are characterized by intense radiation from a compact core, often due to a supermassive black hole. Active galaxies and quasars are considered to be manifestations of AGNs.
Step-by-step explanation:
Active Galactic Nuclei Defined
To define an active galactic nucleus (AGN), we look at the characteristics and activities at the core of certain galaxies that produce an excessive amount of energy. An AGN is usually found in active galaxies, which are distinguished by their bright cores and sometimes by strong emissions across various parts of the electromagnetic spectrum, including radio waves. These galactic centers are highly luminous and can exhibit spectra akin to those of quasars, which are essentially very bright and distant AGNs that were initially mistaken for stars due to their point-like appearance. The exceptional luminosity of an AGN is a result of intense energy generation in a relatively small volume at the galaxy's center.
This activity often correlates with the presence of a supermassive black hole that actively accretes matter, generating radiation across the electromagnetic spectrum.