A process of defining the measurement of a phenomenon that is not directly measurable, though its existence is inferred by other phenomena. Operationalization

Operationalism is based on the intuition that we do not know the meaning of a concept unless we have a method of measurement for it. It is commonly considered a theory of meaning which states that “we mean by any concept nothing more than a set of operations; the concept is synonymous with the corresponding set of operations” https://plato.stanford.edu/entries/operationalism/