Schwarzschild Radius: The Schwarzschild radius is the critical radius around a mass, within which the escape velocity exceeds the speed of light, meaning that nothing, not even light, can escape from this region. This concept is fundamental to the understanding of black holes.
General Relativity: General relativity is the geometric theory of gravitation developed by Albert Einstein, which describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the presence of mass/energy. Schwarzschild's work was based on and contributed to the development of general relativity.
Singularity: In the context of black holes, a singularity refers to a point in spacetime where the gravitational field becomes infinitely strong, and the curvature of spacetime becomes infinite. The Schwarzschild solution predicts the existence of such a singularity at the center of a black hole.