As well as there being no universal measure for the level of scarring, there is also no generally accepted definition of it. Originally, it was stated that certain unstable periodic orbits are shown to permanently scar some quantum eigenfunctions as ℏ ℏ → → 0 , in the sense that extra density surrounds the region of the periodic orbit. However, a more formal definition for scarring would be the following: A quantum eigenstate of a classically chaotic system is scarred by a periodic if its density on the classical invariant manifolds near and all along that periodic is systematically enhanced above the classical, statistically expected density along that orbit. A fascinating consequence of this enhancement is antiscarring. Since there may be strongly scarred states among the eigenstates, the necessity for a uniform average over a large number of states requires the existence of antiscars with low probability in the region of ''regular'' scars. Furthermore, it has been realized that some decay processes have antiscarred states with anomalously long escape times.