An apparent horizon is a term used in the context of general relativity and black hole physics. It is defined as a surface beyond which no light or other forms of radiation can escape to an outside observer. Unlike the event horizon, which is a global feature of a black hole that delineates the boundary beyond which events cannot impact an outside observer, the apparent horizon can be a more localized and dynamic feature.
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An apparent horizon is the outer boundary of the future-trapped region on a chosen spatial slice, under the usual regularity conditions. Its location depends on the slice; it is not the globally defined event horizon. At a regular boundary the outward null expansion vanishes while the inward null expansion is negative. Its area cannot be substituted into every version of the Penrose inequality without the appropriate area and initial-data hypotheses.