event horizon

noun Astronomy.
the boundary around a black hole on and within which no matter or radiation can escape.

Origin:
1970–75

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World English Dictionary
event horizon
 
n
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 & Unabridged 10th Edition
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American Heritage
Science Dictionary
event horizon   (ĭ-věnt')  Pronunciation Key 
A spatial boundary around a black hole inside which gravity is strong enough to prevent all matter and radiation from escaping. The inability of even light to escape this region is what gives black holes their name.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
And interest rates have reached the event horizon of zero.
So the region where the flow changes from subsonic to supersonic is an event
  horizon.
Once you cross the event horizon, you can never escape.
It would only have gravity due to the event horizon.
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