A metal umlaut is a diacritic that is sometimes used gratuitously or decoratively over letters in the names of mainly hard rock or heavy metal bands—for example, those of Blue Öyster Cult, Queensrÿche, Motörhead, the Accüsed, Mötley Crüe and the parody bands Spın̈al Tap and Green Jellÿ.

A diacritic is a glyph added to a letter or to a basic glyph. The term derives from the Ancient Greek διακριτικός, from διακρίνω. The word diacritic is a noun, though it is sometimes used in an attributive sense, whereas diacritical is only an adjective. Some diacritics, such as the acute ⟨á⟩, grave ⟨à⟩, and circumflex ⟨â⟩, are often called accents. Diacritics may appear above or below a letter or in some other position such as within the letter or between two letters.