Umlaut n. : The euphonic modification of a root vowel sound by the influence of a, u, or especially i, in the syllable which formerly followed.

Example:
How do I make it so I can type using German Umlaut when I need them?

Taxonomy
umlaut, ä, ö, diaeresis, ü, vowel, languages, German, diacritic, characters, trema, double dots, umlaut diacritic, English, Unicode, Swedish, diacritic mark, digraph.

Defination from the web:
umlaut and diaeresis characters is not always made. ... Originally, phonological umlaut was denoted in written German by adding an e to the affected vowel, either after the vowel or, in small form, above it. ...