The article deals with terminological definitions, which are elements of the construction of scientific texts. It is substantiated that in such definitions the metalanguage function of the language realizes its potential, therefore these elements are described as metatexts participating simultaneously in the process of text generation and in the explanation of the language. A new approach to the analysis of the units under study is the consideration of “part-whole” relation as a mechanism of their construction. As a result of this analysis the authors identify and describe variants of partitive mechanisms of the construction of terminological definitions functioning in scientific texts.