Active Galaxies are galaxies characterized by certain properties: (1) High Luminosity, (2) Nonthermal Spectra that do not look like the sum of many stellar spectra, (3) Most of the luminosity is in a region of the spectrum other than optical (e.g., radio, UV, Infrared), (4) bright, star-like nucleus, (5) strong emission lines (most), (6) rapid variability, and sometimes (7) radio jets.
The central region of an active galaxy is called an Active Galactic Nucleus (AGN). This is where the energtic activity is concentrated. The active galactic nuclei are believed to contain supermassive black holes that power the nonstellar phenomena associated with active galaxies.
Examples of Active Galaxies include Seyfert Galaxies, Radio Galaxies, and BL Lac objects.
Artist's conception of an accretion disk and jet around a supermassive
black hole in the core of an active galaxy. (Greg Foss; PSC)