The notion of entropy, at first used in the studies of physical systems and then introduced into communication systems, today appears in the analysis of the behaviour of various systems. The authors particularly emphasize the application of the notion of entropy in studying various economic and social phenomena. For this reason, the terms and properties related to entropy have been systematized in this article. In addition, the proofs of many properties have been expanded and complemented and axioms underlying the definition of the entropy of an experimental result, events, random variables, random variable distributions and classes have been formalized. The categorization of entropy terms and properties will enable further applications of the ideas contained in this article in the studies of systems of any type, particularly social and economic systems, which the authors present in other papers.