dieresis

noun pl. -·ses′-sēz′

1. the separation of two consecutive vowels, esp. of a diphthong, into two syllables
2. a mark (¨) placed over the second of two consecutive vowels to show that it is pronounced in a separate syllable: the dieresis is now usually replaced by a hyphen (reënter, re-enter) or simply omitted (cooperate, naive): the mark is also used, as in this dictionary, to show a certain pronunciation of a vowel (ä, ë, ö, ü)
3. Prosody a slight break or pause in a line of verse, resulting when the end of a metric foot coincides with the end of a word

Etymology: LL diaeresis < Gr diairesis, division < diairein, to divide, separate < dia-, apart + hairein, to take: see heresy