Event Horizon

An event horizon, also known as a gravitational boundary is boundary dividing space into a region that can be seen from one that cannot. In the case of a black hole, it is the surface surrounding the region out of which light itself cannot escape. No signal or information from within the event horizon can reach the outside universe. In addition, nothing can prevent a particle from hitting the singularity in a very short amount of proper time once it has entered the horizon. In this sense, the event horizon is a "point of no return".