The boundary separating a black hole’s interior from the observable universe is called the event horizon.
In general relativity, an event horizon is a boundary in spacetime beyond which signals cannot reach distant observers. For a black hole, that means light and matter crossing the horizon cannot return to the outside universe. The horizon is not a solid shell and does not have a material surface.
A distant observer may see infalling light become increasingly redshifted and delayed near the horizon, while the falling object crosses it in its own proper time. The difference comes from how different observers assign coordinates and measure events in curved spacetime.
The event horizon should not be confused with the singularity, which is the central region predicted by classical general relativity, or with the accretion disk, which is hot gas outside the horizon. A black hole can have a horizon even when no bright disk is present.