D defs.my
Entry 1 sense Webster, 1913

umlaut

/(o͞om"lout)/ · um·laut · IPA /ˈʊm.laʊt/
01 n. The euphonic modification of a root vowel sound by the influence of a, u, or especially i, in the syllable which formerly followed.
  1. 1.
    The euphonic modification of a root vowel sound by the influence of a, u, or especially i, in the syllable which formerly followed.(Philol.)