  Four possible scenarios are considered for a transition from a quark-gluon
matter to hadronic matter, and their corresponding correlation signatures are
discussed. Four criteria are highlighted for a definitive experimental search
for a QCD critical point. An old-new experimental measure, the optical opacity
(or its inverse the nuclear attenuation length) is determined, in terms of a
combination of nuclear suppression factors and a measurement of the relevant
fireball length scales. Length scale estimates using either the Hanbury Brown
-- Twiss radii or that of the initial nuclear geometry for measurements of
optical opacity with respect to the reaction plane yield, somewhat
surprizingly, nearly the same nuclear attenuation lenght in 0-5 % most central
200 GeV Au+Au collisions, corresponding to 2.9 $\pm$ 0.3 fm. The necessity and
the possibility of measuring critical exponents is also discussed in the
context of determination of the universality class of the QCD critical point.
Critical opalescence is proposed to locate such a critical point on the QCD
phase diagram, corresponding to a maximum of optical opacity in heavy ion
experiments.
