Merriam-Webster Collegiate Dictionary - diaeresis

 or dieresis  noun  (plural diaereses)  Etymology: Late Latin ~, from Greek diairesis, literally, division, from diairein to divide, from dia- + hairein to take  Date: circa 1611  1. a mark ? placed over a vowel to indicate that the vowel is pronounced in a separate syllable (as in naive or Bronte) — compare umlaut  2. the break in a verse caused by the coincidence of the end of a foot with the end of a word  • diaeretic adjective