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The region, usually described as spherical, marking the outer boundary of a black hole, inside which the gravitational force is strong enough to prevent all matter and radiation from escaping.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Astronomy) astronomy the surface around a black hole enclosing the space from which electromagnetic radiation cannot escape due to gravitational attraction. For a non-rotating black hole, the radius is proportional to the mass of the black hole
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
the boundary around a black hole on and within which no matter or radiation can escape.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.