U+00EF Latin Small Letter I with Diaeresis

The Wikipedia has the following information about this codepoint:

Ï, lowercase ï, is a symbol used in various languages written with the Latin alphabet; it can be read as the letter I with diaeresis, I-umlaut or I-trema.

Initially in French and also in Afrikaans, Catalan, Dutch, Galician, Southern Sami, Welsh, and occasionally English, ⟨ï⟩ is used when ⟨i⟩ follows another vowel and indicates hiatus in the pronunciation of such a word. It indicates that the two vowels are pronounced in separate syllables, rather than together as a diphthong or digraph. For example, French maïs (IPA: [ma.is], maize); without the diaeresis, the ⟨i⟩ is part of the digraph ⟨ai⟩: mais (IPA: [mɛ], but). The letter is also used in the same context in Dutch, as in Oekraïne (pronounced [ukraːˈinə], Ukraine), and English naïve ( nah-EEV or ny-EEV).

In scholarly writing on Turkic languages, ⟨ï⟩ is sometimes used to write the close back unrounded vowel /ɯ/, which, in the standard modern Turkish alphabet, is written as the dotless i ⟨ı⟩. The back neutral vowel reconstructed in Proto-Mongolic is sometimes written ⟨ï⟩.

In the transcription of Amazonian languages, ï is used to represent the high central vowel [ɨ].

It is also a transliteration of the rune ᛇ.