1. A region of space in which mass is packed so densely that (according to Einstein's general theory of relativity) nothing, not even light, can escape.
2. An object whose matter has become so condensed that its gravitational field is striking enough to prevent light escaping from it.
3. In theory, a collapsed object (perhaps a massive star) whose gravitational field is so strong that under most circumstances no light or matter can escape.

The term was invented by the American physicist John Wheeler in 1968 (some sources say 1967). Within a few years, other usages had expanded the phrase for other applications; such as, to describe everything from large budget deficits to gaping legal loopholes.

The "black hole" is also used as a verb. In technological circles, to "black-hole" means to cut off data going to and coming from an address, particularly an address used by a spammer. This usage has been around since about 1997.