Black hole
From Wikiversity
A black hole is a theoretical region of space that has such a strong gravity pull that not even light can escape it. It is called black because it sucks in all light, reflecting nothing, therefore appearing "black". Though it is not possible to see one, a black hole's location can be inferred by observing any celestial bodies that orbit an apparently empty space.
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[edit] Terminology
A black hole is often referred to as any object whose escape velocity exceeds the speed of light.
The escape velocity is the speed at which an object needs to travel so as to just manage to get infinitely far away from a source of gravity before stopping. On the Earth, the escape velocity is equal to 11 km/s, so no matter what the object is, a rocket or a baseball, it must go at least 11 km/s to avoid falling back to the Earth's surface eventually. Light passing by a black hole would be sucked in, it it came close enough.
[edit] Event horizon
The event horizon is a region of spacetime that cannot affect an outside observer in any way. Coming close to the black hole, an objects movement is encouraged to move towards the event horizon. If the object crosses the event horizon, then any possible movement would just pull it deeper.
