Abstract <p>Lack of a unified and commonly accepted standard for describing flaws in performing equipment diagnostics results in that different authors use essentially different terminology for denoting them. In view of this circumstance, difficulties are encountered in attempts to generalize the signs presented in various literature sources, as well as flaw revealing methods. In addition, in many publications on diagnostics, their authors use, at a first glance, similar terminology: “defect,” “damage,” and “flaw.” The term “defect” means each individual nonconformity of products to established requirements; i.e., this term is more relevant to errors in the technology of manufacture, assembling, and repair of a product by the corresponding specialized enterprises, at which the product quality control is performed, so that this term is not quite relevant to equipment damages occurred during its operation. The term “damage” denotes an event which consists in a loss of an object’s perfect state with retaining its up state, and the term “flaw” denotes an object’s state in which it does not meet at least one of the requirements stated by the regulatory technical and/or engineering (design) documentation. The term “flaw” is the most exhaustive one and includes both the cause (the actual damage of parts) and effect (deviation of the parameters monitored and manifestation of this damage). For the purpose of further development of diagnostic systems, it is necessary to elaborate a group of standards regulating the developments in the field of power equipment diagnostics. The article presents—taking a steam turbine as an example—proposals on general classification of damage kinds and on standardization of terms and definitions of main damages.</p>