“electron correlation” is another important concept. It is the property that rises most of the hurdles for finding the exact solution of the quantum system (molecule or solid).
[Optional material: some people wonder at this stage whether there is a connection between exchange and indistinguisble particles. If you are one of them, you may enjoy reading this text from the University of Edinburgh. It is by far the most lucid explanation of this topic I have ever met.]
Why is there a need for a different definition of correlation in classical systems and in quantum systems? Why couldn’t the same definition be used for both? Explain it in your own words on this forum.