di·er·e·sis / dīˈerəsis/ (also di·aer·e·sis) • n. (pl. -ses / -ˌsēz/ ) 1. a mark (¨) placed over a vowel to indicate that it is sounded in a separate syllable, as in naïve, Brontë. ∎ the division of a sound into two syllables, esp. by sounding a diphthong as two vowels.2. Prosody a natural rhythmic break in a line of verse where the end of a metrical foot coincides with the end of a word.