Event Horizon
Definition
The point of no return around a black hole — cross it and not even light can escape back out.
Technical definition
For a Schwarzschild black hole, r_s = 2GM/c². It is a causal boundary, not a physical surface; infalling observers notice nothing special at crossing.
Example
A 10-solar-mass black hole's horizon is ~30 km in radius — a city-sized sphere marking the edge of the knowable.
Common confusion
The event horizon is not a wall of fire. An astronaut falling into a big black hole feels nothing at the horizon — the strangeness is that exit becomes impossible, not that entry hurts.