Final answer:
A black hole is defined as an object with an escape velocity that exceeds the speed of light, rendering it impossible for light to escape. The concept has evolved from early speculations by Michell and Laplace to a critical feature of Einstein's general relativity. An event horizon marks the boundary of a black hole.
Step-by-step explanation:
An object whose escape velocity exceeds the speed of light is known as a black hole. The concept of such an object dates back to the 18th century when John Michell and Pierre-Simon Laplace contemplated the existence of stars from which light could not escape. However, the modern understanding of black holes originated with Einstein's theory of general relativity, which predicted these objects' existence. Black holes are stellar remnants with such immense gravitational fields that not even photons can achieve the velocity necessary to escape. The defining boundary of a black hole is the event horizon, a theoretical boundary where the escape velocity equals the speed of light.
Bodies like the Earth or the Sun also have escape velocities, but they are much less than the speed of light, which allows objects, like spacecraft, to leave their gravitational influence. It's only when a star collapses beyond a certain point, becoming exceptionally dense, that the escape velocity at its surface can surpass the speed of light, trapping everything, including light, inside the event horizon. Thus, black holes are invisible, emitting no light, and anything that crosses the event horizon cannot return.