The article provides a comprehensive analysis of the basic concepts and terms used in the field of psychological diagnostics. The scientific approaches to the formation of terms and conceptual apparatus by which this branch of psychological science describes its subject of study are considered. It is emphasised that the development of science is determined primarily by the formation of terminology, that is, its language as a system of general and specific concepts that are reflected in certain terms and signs. It is stated that the language of psychodiagnostics is a complex, multilevel, integral system, the elements of which are categories, concepts, terms, symbols. It is shown that in the course of the development of psychology, the conceptual apparatus is continuously being improved, definitions are being clarified, and the terminological vocabulary is being enriched. The current state of development of psychodiagnostics, the formation of promising theories of assessment and measurement of individual characteristics have necessitated the clarification of its terminological apparatus, a significant number of concepts, definitions, which are often contradictory, through the use of various approaches and terminological elements. It is emphasised that the conceptual and terminological apparatus of psychodiagnostics should be developed on the basis of modern realities, abandoning outdated approaches to the definition of certain concepts, and that work towards clarifying and unifying terms requires special care and validity. With a view to improving the terminology, the author provides clarifications of some common definitions. Thus, it is proposed to distinguish between the concepts of “qualities” and “traits”, since “quality” corresponds to the Kettellian term “superficial” or “primary” trait and the Allportian term “secondary” or “secondary” disposition, and the concept of “trait” is more generalised, it summarises the existence of a significant number of qualities. It is proposed to understand individual (individual psychological) properties as generalised characteristics of temperament, character, abilities, orientation, i.e. stable mental phenomena which influence human activity, characterise a personality from the psychological and social aspects and are implemented in society. In addition, it is proposed to clearly distinguish between the concepts of “qualities”, “traits”, “features”, to introduce the terms “individual and personal properties and traits”, and to differentiate psychodiagnostic tools for the study of constitutional, individual and personal traits.