dieresis (linguistics)
Definition - Dividing one syllable into two, especially when pronouncing consecutive vowels.

Example -
naïve (pronounced 'ny - eev)

Etymology -
The word derives from the Late Latin diaeresis (from Greek dia, apart + hairein, to take).

Oxford English Dictionary -
Its first citation is from about 1656:
"The figure Diaeresis, whereby one syllable is divided into two parts, as Evoluisse for Evolvisse."
(Blount, Glossographia s.v. Dieretic )

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