The word “correlation” means “mutual dependence” or “interrelation” and the term “electronic correlation” summarises all effects arising from the mutual repulsion of negatively charged electrons. More precisely, it refers to how this repulsion influences the spatial and dynamical motion of the electrons. The concept can be likened to the behaviour of people in a crowded market place: people and electrons have to avoid each other so as not to bump into one other, something that greatly influences their motion.