In Astronomy What Is A Horizon?

From Wikipedia:
In general relativity, an event horizon is a boundary in spacetime, most often an area surrounding a black hole,
beyond which events cannot affect an outside observer. Light emitted
from beyond the horizon can never reach the observer, and any object
that approaches the horizon from the observer's side appears to slow
down and never quite pass through the horizon, with its image becoming
more and more redshifted
as time elapses. The traveling object, however, experiences no strange
effects and does, in fact, pass through the horizon in a finite amount
of proper time.
More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Still other distinct notions include the Cauchy and Killing horizon; the photon spheres and ergospheres of the Reissner-Nordström solution; particle and cosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.