The article studies the relation between the terminological unit and the notion, in particular, different scientific standpoints as to defining notions in scientific and productive activity have been presented. Unlike other words of the language, the term must have a definition, which allows not only to name the notion but define it comprehensively and clearly. The problem of defining as an action that provides for the correct usage of the terminological unit have been developed for a long time and has a centuries-old tradition. However, even today, despite a great number of literature devoted to defining, there are difficulties in evaluation of its role and essence. Therefore, the article analyzes scientific standpoints of well-known linguists both of the past and today concerning the specified problem. We can assert that the term always fixes a distinctly defined notion, which conveys the author’s scientific and technical idea most exactly, contributes to laconism of the statement, is an instrument of scientific thinking and cognition. The term presents a single sign, which corresponds to a single notion. Each phenomenon, each object in scientific or productive activity should have a name of their own and all representatives of the sphere, from which the term was borrowed, should understand the meaning of this name absolutely identically. Unlike the common word, which performs either nominative or definitive function, the term not only names but also logically, thoroughly and exactly — through the definition — defines the notion, gives comprehensive, extensional and at the same time laconic characteristic of an object or a phenomenon. At the same time, it is important that the semantic field of the term would be unidirectional, that is, having only one meaning, which is particularly desirable for terminological system of any science.