Currently linguistic research has concentrated on various aspect of linguistic borrowing of lexical units. At the same time there is no standard terminology describing its process and transformation. These facts highlight relevance of the research carried out in the paper. The article aims to devise a conceptual and terminological apparatus used to describe the processes of retrieving, assimilating or adapting the borrowed lexemes in the language-recipient. Generally the terms “assimilation”, “naturalization”, “integration” and “adaptation” characterize borrowing. The scientific novelty of the study lies in clarification of the terminological apparatus, etymological origin of the terms and semantics in other areas of scientific knowledge. In addition, practical application if the findings is considered. As a result, it is concluded that most optimal term defining the process of borrowing is “assimilation” as it denotes various changes that occur in the process of integration or naturalization of borrowed words into the system of the recipient language. Moreover, the main types of quantitative and qualitative assimilation have been analyzed clarifying terminological aspects of each type. Assimilation has been classified into complete, zero, partial, phonetic, graphic, morphological, syntactic, and semantic one. The term “syntactic assimilation” has been proposed for the first time.