Ḍād (ﺽ ), is one of the six letters the Arabic alphabet added to the twenty-two inherited from the Phoenician alphabet (the others being *ṯāʾ *, *ḫāʾ *, *ḏāl *, *ẓāʾ *, *ġayn *). In name and shape, it is a variant of *ṣād *.Its numerical value is 800 (see Abjad numerals).
The sound it represented at the time of the introduction of the Arabic alphabet is somewhat uncertain, likely a pharyngealized voiced alveolar lateral fricative or a similar affricated sound [ d͡ɮˤ ] or [ dˡˤ ].  In contemporary Arabic, it may represent a pharyngealized voiced alveolar stop , pharyngealized voiced dental stop [ d̪ˤ ] or velarized voiced dental stop [ d̪ˠ ]. 
|Position in word:||Isolated||Final||Medial||Initial|
|Glyph form:||ض ||ـض ||ـضـ ||ضـ |
Based on ancient descriptions of this sound, it is clear that in Qur'anic Arabic ḍ was some sort of unusual lateral sound.    [-1]  Sibawayh, author of the first book on Arabic grammar, explained the letter as being articulated from "between the first part of the side of the tongue and the adjoining molars". It is reconstructed by modern linguists as having been either a pharyngealized voiced alveolar lateral fricative or a similar affricated sound [ d͡ɮˤ ] or [ dˡˤ ].   The affricated form is suggested by loans of ḍ into Akkadian as ld or lṭ and into Malaysian as dl.  However, not all linguists agree on this; the French orientalist André Roman supposes that the letter was actually a voiced emphatic alveolo-palatal sibilant / ʑˤ /, similar to the Polish *ź *.  
This is an extremely unusual sound, and led the early Arabic grammarians to describe Arabic as the لغة الضاد lughat aḍ-ḍād"the language of the ḍād", since the sound was thought to be unique to Arabic.
 The emphatic lateral nature of this sound is possibly inherited from Proto-Semitic, and is compared to a phoneme in South Semitic languages such as Mehri (where it is usually an ejective lateral fricative). The corresponding letter in the South Arabian alphabet is ṣ́, and in Ge'ez alphabet Ṣ́appa ፀ ), although in Ge'ez it merged early on with ṣ.
The reconstruction of Proto-Semitic phonology includes an emphatic voiceless alveolar lateral fricative [ ɬʼ ] or affricate [ t͡ɬʼ ] for ṣ́. This sound is considered to be the direct ancestor of Arabic ḍād, while merging with ṣād in most other Semitic languages.
The letter itself is distinguished a derivation, by addition of a diacritic dot, from ص ṣād (representing /sˤ/).
The standard pronunciation of this letter in modern Standard Arabic is the "emphatic" /d/: pharyngealized voiced alveolar stop , pharyngealized voiced dental stop [ d̪ˤ ] or velarized voiced dental stop [ d̪ˠ ]. 
In most Bedouin influenced Arabic vernaculars ض ḍād and ظ *ẓāʾ * have been merged quite early.  like in the varieties (such as Bedouin and Iraqi), where the dental fricatives are preserved, both the letters are pronounced /ðˤ/.    However, there are dialects in South Arabia and in Mauritania where both the letters are kept different but not in all contexts.  In other vernaculars such as Egyptian the distinction between ض ḍād and ظ *ẓāʾ * is most of the time made; but Classical Arabic ẓāʾ often becomes /zˤ/, e.g. ʿaẓīm (< Classical عظيم ʿaḏ̣īm) "great".   
ض is transliterated as ḍ (D with underdot) in romanization.
When transliterating Arabic in the Hebrew alphabet, it is either written as ד (the letter for / d /) or as צ׳ (*tsadi * with geresh).
|Unicode name||ARABIC LETTER DAD|
|UTF-8||216 182||D8 B6|
|Numeric character reference||ض||ض|