- a galaxy that emits usually large amounts of energy from a very compact central source, such as Seyfert galaxies, radio galaxies, and quasarsAlso called: active galactic nucleus
- A galaxy emitting unusually high quantities of radiation from an active galactic nucleus at its center. Some active galaxies emit more energy in radio wavelengths than they do visible light. Seyfert galaxies are examples of active galaxies. See more at galactic nucleus.