Unadapted borrowing from Late Latindiaeresis(“distribution; division of a diphthong into two syllables”), from Ancient Greekδῐαίρεσῐς(diaíresis, “distribution, division; division of a poetic line when the end of a word and a metrical foot coincide; division of a diphthong into two syllables”),[1] from δῐαιρέω(diairéō, “to divide; to distinguish; to resolve a diphthong or contracted form”) + -σῐς(-sis, suffix forming abstract nouns or nouns of action, process, or result). Δῐαιρέω(Diairéō) is derived from δῐᾰ-(dia-, prefix meaning ‘across; through; in different directions’) (ultimately from Proto-Indo-European*dwís(“doubly, twice; in two”)) + αἱρέω(hairéō, “to grasp, seize, take”) (ultimately from Proto-Indo-European*ser-(“to grasp, seize, take”)).
1843, John Forbes, “Prosody”, in A Double Grammar of English and Gaelic, in which the Principles of Both Languages are Clearly Explained;[…], Edinburgh: W. Whyte & Co.[…], →OCLC, paragraph 7, page 363, column 1:
Synaeresis, the opposite of Diaëresis, is the throwing of two syllables into one; as, Seest for seëst. Looked for look-ed.
1611, Randle Cotgrave, compiler, “Briefe Directions for Such as Desire to Learne the French Tongue. And First, of the Vowels, and Diphthongs.”, in A Dictionarie of the French and English Tongues, London:[…] Adam Islip, →OCLC, signature Nnnn, recto, column 2:
Diæreſis is vvhen tvvo points ouer a vovvell diuide it from another vovvel, as bouë, queuë, read bou-e, queu-e, not bo-ue, que-ue.
1841, William Corry, “Grammatical Terms, with Their Respective Marks and Explanations”, in An English Spelling Book, with Reading Lessons; for the Use of the Parish and Other Schools of New Brunswick, St. John, N.B.:[…] Henry Chubb,[…], →OCLC, page 8:
The Diæresis is used to divide a diphthong into two syllables; as, Creätor.
An 85-year injustice has been rectified at Poets' Corner in Westminster Abbey with the corrected spelling of one of the greatest of all literary names. Reader, it is finally Brontë, not Bronte. An amended memorial to Charlotte, Emily and Anne Brontë was unveiled on Thursday with added diaereses (two dots) that ensure people pronounce it with two syllables. As if it rhymed with Monty, not font. The memorial was installed in 1939 and, for whatever reason, came without the diaereses that the Brontës used.
There are four[sic] manners of performing the Diæreſis: viz. by Cutting, Pricking, Tearing, Dravving, and Burning.
Usage notes
Properly speaking, the terms diaeresis and umlaut are not interchangeable, even though the latter is often a visually identical diacritical mark. As explained in sense 1.1.1, a diaeresis indicates that two consecutive letters (usually vowels) are to be sounded separately. An umlaut over a letter indicates that the sound of a single vowel denoted by that letter is different from the sound denoted by the letter written without an umlaut; for example, in German schon is pronounced /ʃoːn/, while schön is pronounced /ʃøːn/. However, some speakers use the term umlaut to refer to a diaeresis.
In English, it is common to spell words without using diaereses, for example, naive instead of naïve.
(grammar)diaeresis(division of a diphthong into two vowels in consecutive syllables)
AD 98–138, Velius Longus (aut.), T.H.G. Keil (ed.), Liber de orthographia in Grammatici Latini VII (1880), p. 57, ll. 21–28:
sed et quidam in hac quoque scriptione voluerunt esse differentiam, ut pluralis quidem numeri nominativus casus per a et e scriberetur, genetivus vero singularis per a et i, hoc quoque argumentantes, quod diaeresis, sive dialysis illa dicetur, a nominativo plurali non fit, sed ex singulari obliquo, cum dicitur ‘ a u l a i i n m e d i o ’ et // d i v e s e q u u m , d i v e s p i c t a i v e s t i s e t a u r i , // item rei nostrai, faciendai, magnai.
circa AD 384, Ser. Honoratus (aut.), G. Thilo & H. Hagen (eds.), In VergiliiAeneidos commentarii in Servii Grammatici qui feruntur in Vergilii carmina commentarii II (1884), bk vii, l. 464 (p. 160, ll. 1–9):
Fvrit intvs aqvai fvmidvs id est aquae amnis: nec inmerito; nam potest esse et alterius rei amnis, “ut fluvios videt ille cruoris”. Hanc autem diaeresin Tucca et Varius fecerunt: nam Vergilius sic reliquerat “furit intus aquae amnis” et “exuberat amnis”: quod satis asperum fuit. Notandum quod in toto Vergilio non reperiuntur nisi quattuor diaereses, hoc loco, et in tertio ⟨354⟩, ut “aulai medio libabant pocula Bacchi”, et in VI. ⟨747⟩ ut “aurai simplicis ignem”, et in IX. ⟨26⟩ “dives pictai vestis et auri”.