noun: In Arabic script, the vowel point for "i", appearing as a small diagonal line placed below a letter ( ـِ ) and designating a short i /i/. If the Arabic letter ي (yāʾ) immediately follows, it indicates a long ī /iː/.
noun: In a fully vocalised Persian ـِ represents the vowel /e/ and is only used in front of ـی to represent a diphthong, so unmarked دی is /di/ but دِی with the diacritic is /dej/. This usage is different from Arabic.
