Abstract: Ophthalmic terminology is a part of the medical terminology system. The medical terminological system has been studied in sufficient detail, however, to date, questions remain open regarding the synonymy of linguistic units within the ophthalmological terminological system. The purpose of the study is to consider the problem of relations within the ophthalmological terminological system and to prove the existence of a triple series of terms – quasi-synonyms. The object of the study is ophthalmological terminology, the thematic group is "ophthalmological diseases". The relevance of this work is due to the fact that ophthalmological terms with semantic similarity that can both replace each other and function in parallel in speech have not yet been sufficiently studied, in addition, it is necessary to establish what type of synonymy exists between these terms. The method of linguistic experiment used in the work makes it possible to achieve the purpose of the study. The study included 2,535 ophthalmological terms, of which 51 were names of ophthalmological diseases. An experiment was conducted to obtain evidence of the existence of different types of ophthalmic terms. The experiment involved ophthalmologists, nursing staff, patients of the ophthalmological center of Irkutsk (324 people – medical staff, 324 patients). The main conclusion of the study is that there are synonymous relations between terms within the ophthalmological terminological system. The conducted experiment allows us to talk about the existence of a triple set of terms, which confirmed the hypothesis about the functioning, in addition to the generally accepted terminology and professional jargonisms, of "folk" terminology (70% of the total number of terms included in the study). All terms of the triple series, being quasi-synonyms, have semantic similarity, but are not identical, relations of inclusion and intersection arise between them, they exist in the language in parallel. In the future, it is necessary to consider quasi-synonyms of other thematic groups of the ophthalmological terminological system in order to better understand the specifics of their use and the source of their occurrence, as well as whether they can become absolute synonyms and under what conditions.