The relevance of this study lies in its exploration of the problem of the development of the semantics of the term using the example of synonymy in the framework of the modern terminology of art history. The article is focused on three synonymous units: astronomical art, cosmic art, and space art. These lexical units arouse interest as, on the one hand, they claim the status of art history terms, and on the other hand, they are synonymous with each other, which contradicts the requirements for the term system. Revealing the features of the semantics and functioning of these units, and thus tracking the semantic processes that accompany their becoming terms, the authors aim to establish the features of synonymous lexical units in the phase of their formation as terms. The study was carried out according to the case study method, using techniques of definitional analysis, componential analysis, semasiological analysis and contextual analysis, as well as elements of chain and corpus analysis. The research material consists of specialized thematic dictionaries of art history terminology, as well as articles on art history. The concepts under consideration represent an interesting case of the formation of a term and the assignment of an appropriate semantic volume to it. Despite the fact that, initially, astronomical art, cosmic art, and space art entered professional communication as synonyms, they are already breaking out of the scope of synonymy, as evidenced by their functioning in special texts: the concept of cosmic art is wider in meaning than space art, while, according to some sources, astronomical art and space art are in a hyper-hyponymic relationship.