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 travelled to an observer in the age of the universe