dieresis

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Linguistics A mark ( ¨ ) placed over the second of two adjacent vowels to indicate that they are to be pronounced as separate sounds rather than a diphthong, as in naïve.
2. n. Linguistics A mark ( ¨ ) placed over a vowel, such as the final vowel in Brontë, to indicate that the vowel is not silent.
3. n. Poetry A break or pause in a line of verse that occurs when the end of a word and the end of a metrical foot coincide.

Century Dictionary and Cyclopedia

1. n. The separate pronunciation of two vowels usually united as a diphthong; by extension of meaning, separate pronunciation of any two adjacent vowels, or the consequent division of one syllable into two. See dialysis and distraction, 8.
2. n. The sign (¨) regularly placed over the second of two contiguous vowels to indicate that they are pronounced separately; the same sign used for other purposes. The dieresis is used most frequently over e preceded by a or o, in distinction from the diphthongs or digraphs œ and œ. In Greek manuscripts these dots were frequently written over ι and ν beginning a word or a syllable, thus serving also to show that they did not form the close of a diphthong (αι, ει, οι, νι, αν, εν, ον), and their modern use is an extension of this. The employment of the dieresis to mark the full pronunciation of the letters -ed, as termination of the preterit and past participle (for instance, praisëd), though sometimes seen, is not established usage, the acute or grave accent being more common. A similar sign consisting of dots is used merely as a diacritical mark, as in the notation of pronunciation in this book (for instance, ä, ö, ü). A similar mark is used in German to indicate the umlaut. See umlaut.
3. n. In prosody, the division made in a line or a verse by coincidence of the end of a foot and the end of a word; especially, such a division at the close of a colon or rhythmic series. It is strictly distinct from, but often included under, cesura (which see).
4. n. In pathology, a solution of continuity, as an ulcer or a wound.
5. n. In crustaceans, the division in the outer branch of the last pleopods.

Wiktionary

1. n. orthography A diacritic ( ¨ ) placed over the second of two consecutive vowels to indicate that the second vowel is to be pronounced separately from the preceding vowel (as in the girls’ given name of Zoë). It does not indicate a diphthong, but rather that each vowel has its full quality, within the sound-context. Now an uncommon practice in English, but still used in some other languages (e.g. French: haïr, Dutch: ruïne).
2. n. Alternative form of diaeresis.

GNU Webster's 1913

1. n. Same as diæresis.

WordNet 3.0

1. n. a diacritical mark (two dots) placed over a vowel in German to indicate a change in sound