The description of the word you requested from the astronomical dictionary is given below.
Karl Schwarzschild (1873 - 1916) discovered in 1916 that the then new General Theory of Relativity of Albert Einstein predicted that there could be places where the force of gravity becomes infinitely great and the curvature of space-time infinitely large. Such places are called Schwarzschild singularities after him. Nowadays we call the surroundings of such a singularity a black hole. The diameter of a black hole is called the Schwarzschild diameter.