1) The term "black hole" is sometimes used to refer to an imaginary place where objects, files, or funds go when they get lost for no apparent reason.
2) In physics and astronomy, a black hole is a region in time and space within which gravity is so strong that nothing can escape, not even electromagnetic radiation such as visible light. Black holes are thought to surround certain celestial objects.
The idea of a black hole (if not the term itself) is not new. As the intensity of the gravitational field around an object increases, so does the escape velocity. The escape velocity for a celestial mass (such as a star, planet, or moon) is the vertical speed with which an object must be hurled from the surface in order to fly forever beyond the gravitational influence of the mass. If a substantial celestial body such as a star becomes small enough in diameter, the escape velocity at the surface can theoretically exceed the velocity of light. This idea occurred to astronomers even in Isaac Newton's time. Modern astronomers believe they have observed black holes, consisting of stars that have collapsed under their own gravitation after spending their nuclear fuel. Black holes are also believed to exist at the centers of galaxies, including our own.
A black hole produces bizarre effects on time and space. As seen from outside, an object falling into a black hole would approach the so-called event horizon, which is a spherical "one-way membrane" or "Rubicon" surrounding the black hole itself. If the object were a clock, it would seem to run more and more slowly as it approached the event horizon, and would never quite make it inside the black hole. From the reference frame of the falling object, nothing out of the ordinary would take place in the rate at which time passed, and the entry to the black hole would proceed apace, although the gravitational force near the event horizon might tear the falling object apart.
Black holes have been fodder for wild ideas and science-fiction stories since the concept became well known in the mid-1900s. Some scenarios are sensational to the point of madness. For example, suppose a tiny black hole, manufactured for use as a doomsday weapon, were dropped onto the surface of the earth? It would, as the story goes, proceed to devour the planet with unstoppable and phenomenal violence.
Also see a spam-related term, blackhole list.