The event horizon is the boundary around a black hole where the escape velocity equals the speed of light.

The event horizon is not a physical surface but a mathematical boundary. At this radius, the pull of gravity is so strong that the speed needed to escape exceeds the speed of light, which nothing can surpass. This means anything crossing this threshold cannot return or send signals back, making it a one-way barrier. The size of the event horizon depends on the black hole’s mass and varies from one black hole to another. For non-rotating black holes, this boundary is spherical and defined by the Schwarzschild radius.