A black hole is a concentration of mass great enough that the force of gravity prevents anything past its event horizon from escaping it except through quantum tunnelling behaviour (known as Hawking Radiation). The gravitational field is so strong that the escape velocity past its event horizon exceeds the speed of light. This implies that nothing, not even light, inside the event horizon can escape its gravity. It is, however, theorized that wormholes can let one exit a black hole. The term "black hole" is widespread, even though it does not refer to a hole in the usual sense, but rather a region of space from which nothing can return.

The existence of black holes in the universe is well supported by astronomical observation, particularly from studying X-ray emission from X-ray binaries and active galactic nuclei.