The Schwarzschild radius is the distance from a massive object’s center at which its escape velocity equals the speed of light. Squeeze any amount of mass inside that radius and nothing, not even light, can travel fast enough to break free of its gravity again.
It is named after Karl Schwarzschild, who found this solution to Einstein’s equations within weeks of general relativity’s publication in 1915, while serving on the German front during the First World War. For an object with the mass of the sun, the Schwarzschild radius works out to about three kilometers.
Because it marks where escape becomes impossible, the Schwarzschild radius is also called the event horizon. Every object in the universe technically has one, but only the densest, like black holes, are ever actually compressed small enough to sit inside it. Read more from NASA’s Imagine the Universe.