event horizon
The event horizon is the one-way boundary of a black hole: the surface past which no signal, no light, and no traveller can ever return to the outside universe. It is not a wall or a shell you could touch; it is a place defined entirely by the geometry of spacetime. Crossing it is like passing the point on a river above a waterfall where the current is finally too strong to paddle back, except here the current is the flow of the future itself.
Strangely, a person falling in feels nothing special at the moment they cross. For a large black hole the curvature there can be quite mild, so there is no jolt, no barrier, no flash. The horizon is not a local landmark you could detect with instruments in your cabin; it is only meaningful in relation to the outside, because it marks where the path back home has vanished.
From far away, however, the crossing looks very different. An outside observer never actually sees the falling object cross. Its light is stretched to ever longer wavelengths and its image seems to slow, freeze, and fade at the horizon, taking infinitely long to disappear, while in the traveller's own time the crossing happens in a finite, unremarkable instant.
For a non-rotating black hole the horizon sits at the Schwarzschild radius r_s.
The horizon is a feature of spacetime, not a physical membrane. The 'freezing' image an outsider sees is a light-travel and time-dilation effect; the infalling traveller really does cross, and for a large enough black hole survives the crossing unharmed.