Definition from Wiktionary, the free dictionary
ـٌ • (-un)
- ḍamma tanwīn (ضَمَّة تَنْوِين) or ḍammatān (ضَمَّتَان), a nunation (tanwīn (تَنْوِين)) of the vowel u (ـُ - ḍamma), indicating the nominative case in Classical Arabic. In a non-pausal formal situation it's pronounced as "-un". In dialects, relaxed (less formal) Arabic or in pausa, the pronunciation is either shorted to "-u" or ignored altogether, even when written.