Al-Din

Ad-Din (Arabic: الدين‎‎ ad-dīn  "(of) the religion"), a component of some Arabic names, meaning "the religion", e.g. "Sword of the religion".

The Arabic spelling closely transliterated is al-din, but its standard transliteration is ad-din due to the phonological rules involving "sun letter". The first noun of the compound must have the ending -u which, according to the assimilation rules in Arabic, assimilates the following a-, thus manifesting into ud-din in Classical and Modern Standard Arabic. However, all the modern Arabic verniculars 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 od-din.

So 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:

...
Wikipedia