Astron. the spherical boundary surrounding a black hole, within which there is such strong gravity that nothing, not even light, can escape
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.
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.
(plural event horizons)