A horizon is a boundary in spacetime satisfying prescribed conditions. There are several types of horizons that play a role in Albert 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, thus referring to a black hole's boundary and the boundary of an expanding universe * 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