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A horizon is a boundary in spacetime satisfying prescribed conditions.
 | The present page holds the title of a primary topic, and an article needs to be written about it. It is believed to qualify as a broad-concept article. It may be written directly at this page or drafted elsewhere and then moved to this title. Related titles should be described in Horizon (general relativity), while unrelated titles should be moved to Horizon (general relativity) (disambiguation). |  |
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 travelled to an observer in the age of the universe
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