dieresis (n.)

also diaeresis, 1610s, "sign or mark ( ) regularly placed over the second of two contiguous vowels to indicate they are pronounced as two simple sounds," from Late Latin diaeresis, from Greek diairesis "division," noun of action from diairein "to divide, separate," from dia "apart" (see dia-) + hairein "to take" (see heresy).

Meaning "separate pronunciation of two vowels usually united as a diphthong" is from 1650s. In classical prosody, "the slight break in the forward motion of a line that is felt when the end of a foot coincides with the end of a word" [Miller Williams, "Patterns of Poetry"]. Related: dieretic; diaeretic.