The article provides a terminological proposition aimed to complement the system of notions describing interaction of language systems in various cases of communication, when the participants have significant differences in their language background, including those of language acquisition (language learning and teaching). The author proposes to use the term “transidiomacity” to mark the ability of some element of language system to provoke the uncontrolled switches of the language code due to the similarity of the element with some element in another language system. “Transidiomacity” is constructed as a generalizing notion e.g. in its relation to such notion as “interlingual homonymy”, first, because it supposes not only the code switch leading to a communicative confusion (language interference), but also the one leading to better communication due to the borrowing of some means from another system (positive language transfer, intercomprehension), and, second, because it can be applied not only to the units of lexical level, but also to the units of any other level of language system (with the appropriate attributive modification of the term).