  We review the basic definitions and properties of trapped surfaces and
discuss them in the context of Kerr-Vaidya line-element. Our study shows that
the apparent horizon does not exist in general for axisymmetric space-times.
The reason being the surface at which the null tangent vectors are geodesic and
the surface at which the expansion of such vectors vanishes do not coincide.
Calculation of an approximate apparent horizon for space-times that ensure its
existence seems to be the only way to get away with this problem. The
approximate apparent horizon, however, turned out to be non-unique. The choice
of the shear free null geodesics, at least in the leading order, seem to remove
this non-uniqueness. We also propose a new definition of the black hole
boundary.
