What is "critical opalescence"

n. (context physics English) the milky turbidity observed in a fluid as it is slowly cooled to just above its critical point, at which point two clear phases (gas and liquid) form

In 1908 the Polish physicist Marian Smoluchowski became the first to ascribe the phenomenon of critical opalescence to large density fluctuations. In 1910 Albert Einstein showed that the link between critical opalescence and Rayleigh scattering is quantitative 1.