  We propose a formulation of an absorbing boundary for a quantum particle. The
formulation is based on a Feynman-type integral over trajectories that are
confined to the non-absorbing region. Trajectories that reach the absorbing
wall are discounted from the population of the surviving trajectories with a
certain weighting factor. Under the assumption that absorbed trajectories do
not interfere with the surviving trajectories, we obtain a time dependent
absorption law. Two examples are worked out.
