The Collaborative International Dictionary of English v.0.48:

Diaeresis \Di*[ae]r"e*sis\, Dieresis \Di*er"e*sis\ (?; 277), n.;
   pl. Di[ae]reses or Diereses. [L. diaeresis, Gr. ?, fr. ?
   to divide; dia` through, asunder + ? to take. See Heresy.]
   1. (Gram.) The separation or resolution of one syllable into
      two; -- the opposite of syn[ae]resis.
      [1913 Webster]

   2. A mark consisting of two dots [[umlaut]], placed over the
      second of two adjacent vowels, to denote that they are to
      be pronounced as distinct letters; as, co["o]perate,
      a["e]rial.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diaeresis
    n 1: a diacritical mark (two dots) placed over a vowel in German
         to indicate a change in sound [syn: umlaut, dieresis,
         diaeresis]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "diaeresis":
   Alexandrine, accent, accentuation, amphibrach, amphimacer,
   anacrusis, analysis, anapest, anatomy, antispast, arsis, bacchius,
   beat, breakdown, cadence, caesura, catalexis, chloriamb,
   chloriambus, colon, counterpoint, cretic, dactyl,
   dactylic hexameter, dimeter, dipody, dissection, dochmiac, elegiac,
   elegiac couplet, elegiac pentameter, emphasis, epitrite,
   feminine caesura, foot, heptameter, heptapody, heroic couplet,
   hexameter, hexapody, iamb, iambic, iambic pentameter, ictus, ionic,
   jingle, lilt, masculine caesura, measure, meter, metrical accent,
   metrical foot, metrical group, metrical unit, metron, molossus,
   mora, movement, numbers, paeon, pentameter, pentapody, period,
   proceleusmatic, pyrrhic, quantity, resolution, rhythm, spondee,
   sprung rhythm, stress, swing, syzygy, tetrameter, tetrapody,
   tetraseme, thesis, tribrach, trimeter, tripody, triseme, trochee