The term critical point is used in roughly its day-to-day meaning in a variety of contexts, usually where there is some qualitative change in behaviour, such as a phase change with temeprature (e.g. solid/liquid) or crystalisation. When nanoparticle deposition is used in reservoir computing there is a critical point as the susbtrate changes from having no conduction paths to total conduction; at that point it has the most interesting and useful quantum behaviours.
Used on Chap. 7: page 146