The subject of this research is the concept of reduction in the logic and methodology of science. On the one hand, reduction is understood as a relationship between a term and its defining expression within a scientific theory, on the other hand, as a relationship between two theories. Since the expansion of the theory occurs due to the introduction of new terms into its vocabulary with the help of nominal definitions, reduction is an operation opposite to the definition: due to reduction, terms are removed from the dictionary of the theory. Moreover, the theory itself is defined in accordance with the set-theoretic approach as a class of sentences that are closed with respect to derivability. The novelty of the research lies in the fact that it examines the semantic and epistemological aspects of the formal definition of reduction. In particular, the explication of the reduction relation between the two theories is based on the concept of functional equivalence of theories. It has been established that the list of basic terms of the theory can only be specified conventionally. All terms introduced with the help of nominal definitions turn out to be reducible. Consequently, a distinctive feature of a theoretical term is the possibility of its reduction.

Reduction, definition, theory, descriptive term

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