Meaning of Schwarzschild radius

Schwarz'schild ra"dius

Pronunciation: (Ger. shvärts'shilt" Eng. shwôrts'chīld", -shild), [key]
— Astron. Astron.
  1. the radius at which a gravitationally collapsing celestial body becomes a black hole.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.