An event horizon is a lightlike surface in spacetime which divides spacetime into two regions: that which can be observed, and that which cannot.
In the case of a black hole, the event horizon is that surface surround the region out of which light itself cannot escape. No signal or information from within the event horizon can reach the outside universe. For a nonrotating black hole, the horizon is located at the Schwarzschild radius, corresponding to
Rs = 2GM/c2