An event horizon is the boundary surrounding a black hole beyond which nothing can escape, not even light. It marks the point where the gravitational pull becomes so strong that escape velocity exceeds the speed of light. Once an object crosses this boundary, it cannot return to the outside universe. The concept of an event horizon is crucial in understanding the nature of black holes, which are formed from the remnants of massive stars after they collapse. The event horizon is not a physical surface but rather a mathematical boundary that defines the limits of a black hole's influence in space, impacting theories in general relativity.