The event horizon is the boundary around a black hole beyond which escape is impossible. In general relativity, crossing it means that every future-directed path leads deeper into the black hole, including paths followed by light.
It is not a solid surface and does not usually produce a visible flash when an observer crosses it. For a simple, nonrotating black hole, the event horizon lies at the Schwarzschild radius, which increases in direct proportion to the black hole’s mass.
The event horizon is often confused with the singularity. The horizon is a causal boundary, while the singularity is the region where the classical theory predicts extreme curvature and where current physics is incomplete. The dark central region in a black-hole image is also not simply a photograph of the horizon; it includes effects from photon paths and glowing material around it.