Purpose. The author considers the problem of establishing a correlation between lacunarity and neologization of the language system in the diachronic aspect using methods of semantic, contrastive, contextual, corpus and quantitative analysis. The material for the study is Russian writers’ epistolary texts of the second half of the 19th century. The author specifies the meaning of the term lacuna in relation to diachrony: a lacuna is an empty cell in the lexical system of a language, revealed retrospectively by detecting means that compensate for it. Results. The author’s classification of lacunae is based on a semiotic criterion: conceptual lacunae are empty cells in the lexical system, which are characterized by the absence of both the signifier and the signified (concept); lexical lacunae are empty cells in the lexical system, which lack a signifier, but have a non-verbalized signified that exists in public or individual consciousness. Conclusion. Extralinguistic and intra-linguistic factors determined the formation and compensation of communicatively significant lacunae in the lexical system at the level of language and speech: changes in the socio-political, economic, cultural life of Russia; interlanguage contacts; the need for nomination or re-designation of the concept; the laws of linguistic regularity, analogy, generalization and differentiation, economy, the tendency to expressiveness.