The study of idiom as a separate unit by Ukrainian scholars marks the beginning of its general linguistic interpretation. In this regard, it is established that the completeness of fixing its linguistic data (e.g. corpus approach), verification of linguistic fact and modern methodological approach to it based on models and generalizations will provide from specialized to interscientific interpretation of idiom in the system of language knowledge. Idiom as a language system has a number of features - structure, normative, systematic organization, etc. - similar to the language system in general, which is determined by the internal laws of the idiom, dialect norm or immanent essence of language. The idiom system organizationally consists of units – the idiomеms - significant units of speech as linguistic facts. To study them, it is appropriate to allocate a special section of dialectology - idiomology, which is devoted to a comprehensive study of speech as a language system with all its conceptual apparatus in metalanguage linguistics, methodology and empirical fund. The fixation of idiom data cannot be differential or in some way limited by other factors. The scientific approach is provided only by the completeness of fixing the vocabulary of the dialect in a certain period. Based on substance, the structure of the idiom system is a comprehensive network of relations between all its elements at different language levels (phonetic, grammatical, lexical). The structure of idiom as a language system can be realized in the description of speech linguistics in the context of monodialectal research, which is, on the one hand, a way to justify its status as a unit of dialect microsystem, and on the other - to clarify its ontological features and pragmatics of speech as a language system in metalanguage linguistics.