The article described the early history of the linguistic term Umlaut in the terminological system of the German language. The study relied on the methods of historical, linguistic, definitional, and semantic analyses. The research covered German grammars, grammatical treatises, and dictionaries published in Germany between the early XVI century and the mid XIX century. The author traced the early evolution of the phonetic status of umlaut from a diphthong to a semi-diphthong: it was often believed to be a combination of two mutually pronounced sounds. In the first half of the XVII century, the status of umlaut began to change. T. Olearius (1630) referred umlauts to simple sounds. However, T. Olearius was not supported by his contemporaries. J. Schottel (1641) ascertained the ability of umlaut to realise a distinctive function. In spite of this, umlaut remained a diphthong up to the last quarter of the XVIII century. The term Umlaut was introduced into linguistic usage only in 1774 by F. Klopstock, who understood it as any change of the root vowels, regardless of the reasons for this alternation. The final status of the umlaut was determined when linguists came to the unequivocal conclusion that umlauts were simple vowels that required a single articulatory effort to be realized in speech. The author also analysed the five ways of umlaut graphics, as well as the names that denoted the diacritical marks. Umlaut received its final terminological status from J. Grimm in 1819.