The authors have focussed attention on a number of concepts widely used by research workers but needing clarification and more careful definition, like metastable states, clusters, supercooling, spinodal decomposition. The material in this article is of great significance to all who are interested in phase transitions, but it has not in the past been given the attention it deserves. Since the authors have successfully undertaken the huge task of covering the relevant literature and describing its content concisely and coherently, this defect may perhaps be remedided in the near future.