Closed trapped surfaces are a concept used in black hole solutions of general relativity[1] which describe the inner region of an event horizon.
The boundary of the union of all trapped surfaces around a black hole is called an apparent horizon.
with restricted bounds, their area tending to decrease locally along any possible future direction and with a dual definition with respect to the past.
Trapped null surfaces are used in the definition of the apparent horizon which typically surrounds a black hole.
More formally, if every null congruence orthogonal to a spacelike two-surface has negative expansion, then such surface is said to be trapped.