The surface of a black hole. For a non-rotating black hole, it is a spherical boundary at the black hole's Schwarzschild radius where the escape velocity becomes equal to the speed of light, so no events occurring within it can be seen from outside. However, the effects of the black hole's powerful gravitational field can still be felt outside the event horizon. For a rotating black hole, the event horizon is elliptical (see Kerr Black Hole).
Subjects: Astronomy and Astrophysics.