Physical horizons

boundary of a region of spacetime from which one cannot escape once entered

the maximum distance from which light from particles could have traveled to the observer during the age of the universe

light-like boundary of the domain of validity of a Cauchy problem in general relativity

horizon appearing on a cosmological scale

boundary in spacetime, defined with respect to the external universe, events inside which cannot affect an external observer

surface that is the boundary between light rays that are directed outwards and moving outwards, and those directed outward but moving inward