active galactic nucleus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A galactic nucleus that emits more radiation than can be accounted for solely by light from the stars it contains.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy A compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum.