In linguistics the term Umlaut is used in a variety of closely related ways, some narrower, some broader. These are discussed in several different articles.

• Germanic umlaut - in its original, narrowest sense, Umlaut is the fronting of vowels in a Germanic language caused by assimilation to an original front vowel in the following syllable, especially viewed diachronically.
  • Umlaut (diacritic) - derived from this, the umlaut diacritic (a pair of dots above a vowel, as <ä> in "doppelgänger") is a method of representing the synchronic results of Germanic Umlaut in German spelling, and subsequently in other languages which borrowed the symbol.
• I-mutation - used more broadly, the term umlaut is sometimes used to refer to the same phenomenon in any language, including non-Germanic languages. A less controversial term is "i-mutation".
  • Affection (linguistics) - an example of this in another language family is the parallel phenomenon in Celtic languages, usually referred to as "affection".
• Vowel harmony and metaphony - more broadly still, some writers use the word umlaut to refer to any example of metaphony, or "regressive vowel harmony".
  • A-mutation - an example of this is a-mutation, the velarisation of vowels caused by an original back vowel in a following syllable, which is occasionally referred to as a-umlaut.
• Apophony - viewed synchronically, umlaut is an example of apophony or vowel alternation, although the use of the term as a synonym for apophony is erroneous.
  • Indo-European ablaut - in particular, the confusion of umlaut with ablaut is is common and erroneous.

• Outside the field of linguistics, the umlaut as a sign inspiring subcultures:
  • Heavy metal umlaut - the umlaut diacritic has been used for sensational spellings in the rock music scene.