A language's vocabulary is enhanced by both words and phraseological units. Phraseological units are word groups that are established and cannot be created during speech. They are collected in specialized dictionaries. Similar to words, phraseological units are one component of a sentence and convey a single idea. Beyond their stylistic value, idioms and set phrases provide semantic compactness, metaphorical reinterpretation, and cultural embeddedness, making them essential tools for naming and conceptualizing reality. This article explores the structural stability and flexibility of phraseological units, particularly the phenomena of interchangeable adjectives, drawing on the works of academics like V.V. Vinogradov and V.N. Telia. The study emphasizes the significance of phraseological units for comprehending how language reflects society, history, and philosophy by stressing their role in communication, cognition, and cultural transmission.