The analysis of the semantics and semantic oppositions of terminological units is one of the most productive ways to understand the systemic nature of terminological phenomena. The article is devoted to the study of semantic features of English- and Russian-language musical terminological units, namely, to the identification of the semantic structure of terms and semantic oppositions that arise between them. Within the semantic structure of musical terms, in the aggregate of their connections and relationships that form the musical terminological system, the core, semantic periphery and additional semantic area are distinguished and classified according to the presence of these components into uninuclear proper, uninuclear with the periphery and uninuclear with the periphery and additional semantic area. At the first stage, a selection of musical terms in modern English and Russian has been compiled, at the second stage, the semantic structure of terminological units has been analyzed, and at the third stage, the key aspects of nomination have been identified, according to which explicit and implicit oppositions are being formed. The material for the study comprises a selection of the English- and Russian-language musical terminological units with the total volume of 1000 units (500 units of each language), selected by the method of continuous selection from electronic and printed sources, including dictionaries and glossaries, musical notations, monographs, textbooks and manuals, web portals on music topics. The research methods of the English- and Russian-language musical terminological units include the method of continuous selection, methods of component and definition analysis, the method of linguacognitive modelling, as well as techniques of quantitative analysis. Within the framework of the study, three groups of musical terms have been identified depending on their semantic structure: the first group includes uninuclear terminological units with the semantic periphery, the second group includes uninuclear terms proper, and the third group is formed by uninuclear terms with the periphery and additional area of semantics. The first group is predominant in terms of quantity, the number of terms in which is almost 2.5 times more than in the second one, and the third group is represented by a small number of terminological units. There are at least 6 frequent and 12 less frequent key aspects of nomination within the musical terminology. The frequent key aspects of nomination with the help of which explicit oppositions are formed are: «Minor / major», «Volume / pitch», «Direction», «Location (locus)», «Singularity / Multiplicity», «Quantity». Less frequent aspects of nomination are: «Weakness / strength», «Perfection / imperfection», «Colour», «Presence / absence of sound», «Structure», «Quantity / Completeness / Size», «Genre of music», «Material», «Recommendations for performing a piece of music», «Role», «Style of playing an instrument», «Type of musical instrument».