dieresis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun A mark ( ¨ ) placed over a vowel, such as the final vowel in Brontë, to indicate that the vowel is not silent.
• noun 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.

from The Century Dictionary.

• noun In crustaceans, the division in the outer branch of the last pleopods.
• noun 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.
• noun 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.
• noun 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).
• noun In pathology, a solution of continuity, as an ulcer or a wound.

from the GNU version of the Collaborative International Dictionary of English.

• noun Same as diæresis.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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).
• noun Alternative form of diaeresis.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a diacritical mark (two dots) placed over a vowel in German to indicate a change in sound