συναίρεσις

English (LSJ)

εως, ἡ,
A taking together or drawing together, ἡ τῶν ἄκρων εἰς ταὐτὸ συναίρεσις Longin.10.3; ς. (sc. καρπῶν) Ath.11.489f; contraction, closing, τῶν διοδευθησομένων Χωρίων Sor.2.59.
2 contraction of front, opp. αὔξησις, Ascl.Tact. 11.7; shortening, reduction of an estimated distance, Ptol.Geog.1.8.3; συναίρεσις τοῦ μήκους (of a fractured limb) Sor.Fract.19 (συνερεισις cod.).
3 aggregation, Dam.Pr.96; synthesis, ib.277,280; concentration, τῆς νοερᾶς οὐσίας Simp. in Ph.635.32; generalization, opp. διαίρεσις, Elias in Porph.76.19.
4 in Gramm., synaeresis, syneresis, whereby two vowels are not changed, but coalesce into a diphthong, as ὀϊστός, οἰστός, opp. διαίρεσις, Quint.Inst.1.5.17, A.D.Adv.132.25; but also contraction, as of κύημα to κῦμα, Gal.6.642.

German (Pape)

[Seite 997] ἡ, das Zusammennehmen. Bei Gramm. die Zusammenziehung zweier Vokale in einen Diphthong, eigtl. dann, wenn sie keine Veränderung erleiden; aber auch allgemein z.B. εα in η, Schol. Ar. Ach. 10.

French (Bailly abrégé)

εως (ἡ) :
1 action d'assembler;
2 récolte;
3 resserrement, contration ; t. de gramm., par opp. à διαίρεσις : synérèse, ou diphtongaison, prononciation de deux voyelles en une seule émission de voix (= synizèse).
Étymologie: συναιρέω.

Russian (Dvoretsky)

συναίρεσις: εως ἡ
1 собирание, соединение, сопоставление (τινος πρός τι Plut.);
2 грам. синэресис (слитное произношение смежных гласных, дифтонгизация, напр., ει из εϊ, οι из οϊ).

Greek (Liddell-Scott)

συναίρεσις: -εως, ἡ, τὸ λαμβάνειν ἢ ἑλκύειν πρὸς τὸ αὐτό, ἡ τῶν ἄκρων εἰς ταὐτὸ σ. Λογγῖν. 10. 3· σ. καρπῶν Ἀθήν. 489F, Πλούτ. 2. 924F. ΙΙ. συστολή, Πτολ. 1. 8, 4, Εὐστ. Πονημάτ. 143. 43. 2) παρὰ γραμμ., ἡ συγχώνευσις δύο φωνηέντων ἢ φωνήεντος καὶ διφθόγγου ἐν μιᾷ λέξει εἰς μακρὸν φωνῆεν ἢ δίφθογγον, οἷον, γέα, γῆ, ἀληθέϊ, ἀληθεῖ, τιμάεις, -ᾶς, ἐν ἀντιθέσει πρὸς τὸ διαίρεσις, Κυντιλ. 1. 5, 17.

Wikipedia EN

In linguistics, synaeresis (/sɪˈnɛrəsɪs/; also spelled syneresis) is a phonological process of sound change in which two adjacent vowels within a word are combined into a single syllable.

The opposite process, in which two adjacent vowels are pronounced separately, is known as "diaeresis".

For any given word, speakers generally hold a traditional view about the standard pronunciation of that word. When realized in a careful reading style, each particular word is associated with this single, standard phonetic form. However, each word also possesses multiple non-standard or reduced phonetic forms which are produced in a greater range of contexts. These multiple variations in the pronunciation of a single word are referred to as allophonic variants. To classify one of these other forms as an allophonic variant of a word means that pronouncing the word in this way will not change the intended meaning of the word.

Synaeresis is one of various phonological processes in which segments of words or phrases are lost. The general term for a loss of sound segments in the field of linguistics is known as "elision". Other types of elision include the processes of apheresis, syncope, apocope, synizesis, and synaloepha.

Synaeresis comes from Greek συναίρεσις (synaíresis), a "contraction", a "taking or drawing together", from συναιρέω (synairéō), "(I) contract", "(I) grasp or seize together", derived from σύν, "with", and αἱρέω, "(I) grasp, seize". Semantically, it is easy to understand how this term evolved historically. The term eventually becamer applied to a process in which vowels are taken or drawn together.