Accretion disk – structure (often a circumstellar disk) formed by diffused material in orbital motion around a massive central body, typically a star. Accretion disks of black holes radiate in the X-ray part of the spectrum.
Black hole thermodynamics – area of study that seeks to reconcile the laws of thermodynamics with the existence of black hole event horizons.
Schwarzschild radius – distance from the center of an object such that, if all the mass of the object were compressed within that sphere, the escape speed from the surface would equal the speed of light.
M-sigma relation – empirical correlation between the stellar velocity dispersion of a galaxy bulge and the mass M of the supermassive black hole at
Event horizon – boundary in spacetime beyond which events cannot affect an outside observer.