- one of over a thousand known extragalactic objects, starlike in appearance and having spectra with characteristically large redshifts, that are thought to be the most distant and most luminous objects in the universe.
- any of a class of extragalactic objects that emit an immense amount of energy in the form of light, infrared radiation, etc, from a compact source. They are extremely distant and their energy generation is thought to involve a supermassive black hole located in the centre of a galaxy
