A horizon is a boundary in spacetime satisfying prescribed conditions.
There are several types of horizons that play a role in Einstein's theory of general relativity:
Absolute horizon, a boundary in spacetime in general relativity inside of which events cannot affect an external observer
Event horizon, a boundary in spacetime beyond which events cannot affect the observer
Apparent horizon, a surface defined in general relativity
Cauchy horizon, a surface found in the study of Cauchy problems
Cosmological horizon, a limit of observability
Killing horizon, a null surface on which there is a Killing vector field
Particle horizon, the maximum distance from which particles can have travelled to an observer in the age of the universe
