The global distribution of the environment-related terms in many scientific and/or technical fields demands a thorough analysis of these special lexical items at various stages of their development: formation, application and standardization. The terminology in the thematic field “environment and ecology” is dynamic and thus open to changes. It also reflects changes in all related systems of terms. The dynamically developing terminology of the thematic field “environment and ecology” is characterised by the existence of intra-field synonymy. The texts in the field of ecology and environment protection abound in a great deal of various types of terms with the high degree of substitutability and/or nearly perfect sameness of meaning. These elements (synonyms, variants and doublets) are always applied for particular purposes, and sometimes point to the frames of special knowledge using the sources of general language. The present paper deals with the analysis of the notion of synonymy in scientific and technical language. The author of the article considers different approaches to synonymy classification and analyses two types of partial synonyms in the thematic field “environment and ecology”, providing empirical examples in English and Latvian.