(), is one of the six letters the Arabic alphabet added to the twenty-two inherited from the Phoenician alphabet (the others being , , , , ). In name and shape, it is a variant of .
Its numerical value is 800 (see Abjad numerals).

In Modern Standard Arabic and many dialects, it represents an "emphatic" , and it might be pronounced as a pharyngealized voiced alveolar stop , pharyngealized voiced dental stop  or velarized voiced dental stop . 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  or . One of the important aspects in some Tihama dialects is the preservation of the emphatic lateral fricative sound , this sound is likely to be very similar to the original realization of ḍād, but this sound () and  are used as two allophones for the two sounds ḍād  and ẓāʾ .

Origin
Based on ancient descriptions of this sound, it is clear that in Qur'anic Arabic ḍ was some sort of unusual lateral sound. 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  or . 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 , similar to the Polish ź.

This is an extremely unusual sound, and led the early Arabic grammarians to describe Arabic as the  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 Soqotri, but also in Mehri where it is usually an ejective lateral fricative. 
The corresponding letter in the  South Arabian alphabet is  , and in Ge'ez alphabet   ፀ), although in Ge'ez it merged early on with . 

The reconstruction of Proto-Semitic phonology includes an emphatic voiceless alveolar lateral fricative  or affricate  for . This sound is considered to be the direct ancestor of Arabic , while merging with  in most other Semitic languages.

The letter itself is distinguished a derivation, by addition of a diacritic dot, from ص ṣād (representing /sˤ/).

Pronunciation

The standard pronunciation of this letter in modern Standard Arabic is the "emphatic" : pharyngealized voiced alveolar stop , pharyngealized voiced dental stop  or velarized voiced dental stop .

In most Bedouin influenced Arabic vernaculars  ḍād and  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 . 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  ḍād and  ẓāʾ contrast; but Classical Arabic ẓāʾ becomes , e.g. ʿaẓīm  (< Classical  ʿaḏ̣īm ) "great".

"De-emphaticized" pronunciation of both letters in the form of the plain  entered into other non-Arabic languages such as Persian, Urdu, Turkish. However, there do exist Arabic borrowings into Ibero-Romance languages as well as Hausa and Malay, where ḍād and ẓāʾ are differentiated.

Transliteration
 is transliterated as ḍ (D with underdot) in romanization. The combination ⟨dh⟩ is also sometimes used colloquially. In varieties where the Ḍād has merged with the Ẓāʾ, the symbol for the latter might be used for both (eg. ⟨⟩ 'to stay' and ⟨⟩ 'to be lost' may both be transcribed as  in Gulf Arabic).

When transliterating Arabic in the Hebrew alphabet,  it is either written as  (the letter for ) or as  (tsadi with geresh), which is also used to represent the /tʃ/ sound. The Arabic letters   and   share the same Semitic origin with the Hebrew tsadi.

In Judeo-Arabic orthography, it has been written as (tsadi with holam), emulating Arabic orthography, where the letter is created by adding a dot to  .

Unicode

See also
Arabic phonology

References

Arabic letters
Ge'ez language