event horizon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Physics, Astron.) the boundary surface surrounding a black hole, from outside of which nothing inside can be observed, because nothing inside that surface, even light, can escape beyond it. See also black hole and escape velocity.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astrophysics The gravitational sphere of a black hole within which the escape velocity is greater than the speed of light, causing time to practically stop.
• noun A point of no return, by analogy to the astronomical usage