An event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In the context of black holes, the event horizon is the point of no return, where the gravitational pull is so strong that nothing, not even light, can escape its grasp. It marks the boundary between the observable universe and the region from which no information or signals can reach an external observer. The concept of the event horizon plays a central role in understanding the properties and behavior of black holes in general relativity.