Ad-Din (Arabic: الْدِّين pronounced as /ar/, "(of) the religion/faith/creed") is a suffix component of some Arabic names in the construct case, meaning 'the religion/faith/creed', e.g. Saif ad-Din (Arabic: سيف الدّين, "Sword of the Faith"). Varieties are also used in non-Arabic names throughout the Muslim world, It is used as a family name-suffix by some royal Muslim families, including the imperial Seljuks, Walashmas, Mughals, and the noble Alvi Hyderabadi families.
The Arabic spelling in its standard transliteration is . Due to the phonological rules involving the "sun letter", the Arabic letter is an assimilated letter of the Arabic definite article . This leads to the variant phonetic transliteration . The first noun of the compound must have the ending -u, which, according to the assimilation rules in Arabic (names in general are in the nominative case), assimilates the following a-, thus manifesting into in Classical and Modern Standard Arabic. However, all modern Arabic vernaculars lack the noun endings. Thus, the vowel of the definite article in them is pronounced in full as either a or e (the latter mostly in Maghreb and Egypt). At the same time, the Arabic short vowel u is rendered as short o in Persian, thus . In practice, romanizations of Arabic names containing this element may vary greatly, including:
Examples of names including this element are:
In modern times in English-speaking environments, the name Uddin has sometimes been used as if it was a separate surname. An example is: