In general relativity, an absolute horizon is a boundary in spacetime, defined with respect to the external universe, inside which events cannot affect an external observer.
An absolute horizon is thought of as the boundary of a black hole.
Examples of asymptotically flat spacetimes include Schwarzschild and Kerr black holes.
The particular feature of asymptotic flatness which is needed is a notion of "future null infinity".
The disadvantage is that it requires the full history (all the way into the future) of the spacetime to be known, thus making event horizons unsuitable for empirical tests.