Umlaut
"Umlaut" refers to a historical sound shift in German. In German, umlauts are found as ä, ö and ü. The name is used in some other languages that share these symbols with German or where the Latin spelling was introduced in the 19th century, replacing marks that had been used previously. The phonological phenomenon of umlaut occurred historically in English as well (man ~ men; full ~ fill; goose ~ geese) in a way cognately parallel with German, but English orthography does not write the sound shift using the umlaut diacritic. Instead, a different letter is used.
Read more about this topic: Diaeresis (diacritic)