Simply put, a black hole is a region of space-time where gravity is so strong that nothing can escape from it. This includes light, which travels at the cosmic speed limit of 186,000 miles per second. In a stroke of descriptive genius, American physicist John Wheeler coined the term "black holes" for these objects.
Although black holes lie at the frontier of astronomy, the idea such objects might exist stretches back more than 200 years. In the late 1700s, British professor John Michell and French astronomer and mathematician Pierre Simon, Marquis de Laplace, advanced the idea of what Laplace called "dark bodies." Using Newton's concepts of light and gravity, they reasoned that the gravitational pull of a massive star could grow large enough to prevent light from escaping. Unfortunately, Newton's theory could not describe what happens when gravity grows that strong.
Enter Albert Einstein. In 1916, his general theory of relativity put black holes on a firm footing. Using general relativity, which treats gravity as a distortion of space-time, physicists were able to describe black holes in gory detail. A key feature of a black hole is its radius. Scientists call this the "event horizon" because it marks the edge beyond which light cannot escape. Any event taking place inside the event horizon can never be glimpsed from outside. At the center of a black hole lies a point of infinite density called a "singularity."