Diaeresis (diacritic)
The diaeresis, umlaut, or more generically trema is a diacritic that consists of two dots ( ¨ ) placed over a letter, most commonly a vowel. When that letter is an i or a j, the diacritic replaces the tittle: ï.
Strictly speaking, the words diaeresis and umlaut refer to two distinct phonological phenomena. The diaeresis is used to denote the phenomenon also known as diaeresis ( /daɪˈɛrɨsɨs/ dy-ERR-ə-səs), or hiatus, in which a vowel letter is not part of a digraph or diphthong. The umlaut ( /ˈʊmlaʊt/ UUM-lowt) refers to a sound shift. The two diacritical uses originated separately, with the diaeresis being considerably older. Nevertheless, in modern computer systems using Unicode, the umlaut and diaeresis diacritics are identical: ⟨ä⟩ represents both a-umlaut and a-diaeresis.
The two dots are also used as a diacritic in other cases, where they are neither diaeresis nor umlaut.
Read more about Diaeresis (diacritic): Etymologies, Diaeresis, Umlaut, Other Uses