In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer.
In layman's terms, it is defined as the shell of "points of no return", i.e., the points at which the gravitational pull becomes so great as to make escape impossible, even for light.
An event horizon is most commonly associated with black holes.
Light emitted from inside the event horizon can never reach the outside observer.
Likewise, any object approaching the horizon from the observer's side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses.
This means that the wavelength is getting longer as the object moves away from the observer. The traveling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time.
NOTE: Above information has been taken from wikipedia and/or official websites of topics.
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