What is the definition of a black hole?

A black hole is a region of space from which nothing, not even light, can escape due to the intense gravitational attraction exerted by the matter contained therein.
Black holes can be formed by the catastrophic end of a very massive star that implodes after exploding as a supernova, or by a slow accumulation of matter or external pressure.
According to the theory of general relativity, any body whose mass is compressed to a small enough radius becomes a black hole.
The spherical surface surrounding a black hole at which the escape velocity equals the speed of light is known as the event horizon.