A trapped surface is a closed spacelike codimension-two surface whose two future-directed orthogonal null congruences both have negative expansion.
An anti-trapped surface is the time reverse of a trapped surface: both future-directed orthogonal null expansions are positive.
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A trapped surface is a concept in the field of general relativity, specifically in the study of black holes and gravitational collapse. It refers to a two-dimensional surface in spacetime that has certain properties related to the behavior of light rays. In more technical terms, a trapped surface is defined as a surface such that all light rays emitted orthogonally (perpendicular) to the surface are converging.