  A marginally trapped surface in the four-dimensional Minkowski space is a
spacelike surface whose mean curvature vector is lightlike at each point. We
associate a geometrically determined moving frame field to such a surface and
using the derivative formulas for this frame field we obtain seven invariant
functions. Our main theorem states that these seven invariants determine the
surface up to a motion in Minkowski space.
  We introduce meridian surfaces as one-parameter systems of meridians of a
rotational hypersurface in the four-dimensional Minkowski space. We find all
marginally trapped meridian surfaces.
