Black hole
In 1783, an English clergyman named John Michell wrote that it might be possible for something to be so heavy you would have to go at the speed of light to get away from its gravity. Gravity gets stronger as something gets more massive. For a small thing, like a rocket, to escape from a larger thing, like Earth, it has to escape the pull of the Earth's gravity or it will fall back. The speed that it must travel  to get away is called escape velocity. Bigger planets (like Jupiter) and stars have more mass, and have stronger gravity than Earth. Therefore, the escape velocity would need to be much faster. John Michell thought it was possible for something to be so big that the escape velocity would need to be faster than the speed of light, so even light could not escape. In 1796, Pierre-Simon Laplace wrote about the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).