1) AGN (active galactic nucleus): It is a small region at the centre of a galaxy that emits a prodigious amount of energy in the form of radio, optical, X-ray, gamma radiation or high speed particle jets.
2) QSO (Quasistellar objects): An active galactic nucleus which is so distant that its stellar systems cannot be resolved. Quasistellar objects show broadened redshifted spectral lines. 5-10% of QSOs are radio loud, and are referred to as Quasars.
3) Seyfert Galaxies:
a. Seyfert 1 Galaxy: Active galaxies which are brightest at UV and X-ray wavelengths. They show broad lines and vary in brightness by up to 50% in a few months.
b. Seyfert 2 Galaxy: Active galaxies which are brightest at infrared wavelengths. They show narrow lines and appear to be cool and dusty.
4) Black Hole: A massive astrophysical object that is theorized to be created from the collapse of a neutron star. The gravitational forces are so strong in a black hole that they overcome neutron degeneracy pressure and, roughly speaking, collapse to a point (known as a singularity). Even light cannot escape the gravitational pull of a black hole within the black hole’s so-called Schwarzschild radius.
Uncharged, zero angular momentum black holes are called Schwarzschild black holes. Uncharged nonzero angular momentum black holes are called Kerr black holes. Nonspinning charged black holes are called Reissner-Nordström black holes. Charged, spinning black holes are called Kerr-Newman black holes. The black hole no hair theorem shows that mass, charge, and angular momentum are the only properties which a black hole can possess.
5) LINER (low-ionization nuclear emission-line region): A type of gaseous region common in the centers of many kinds of galaxies. Some of these have been shown to be low-luminosity active galactic nuclei, perhaps an extension of Seyfert activity to the lowest levels, implying that the whole phenomenon of nuclear activity occurs in a significant fraction of bright galaxies. A LINER’s spectrum is dominated by emission lines from low ionization states, such as OII, NII, and SII, with only weak lines from highly ionized states.
Reference: Eric Wolfram’s World of Astronomy. (Scienceworld.wolfram.com)