Wiktionary
n. A diacritic used in the Devanagari script to describe modern sounds borrowed from languages not native to Devanagari, mainly Urdu (Arabic, Farsi) and English.
Wikipedia
Nuqtā ( Hindi-Urdu नुक़्ता, نقطہ, from the Arabic nuqta نقطة "dot"), also spelled Nuktā, is a term for a diacritic mark introduced in Devanāgari (and some other Indian scripts) used to represent sounds from other languages which do not have a native character. It takes the form of a dot placed below a character. Also, in the Urdu script, there "are some letters in Urdu that share the same basic shape but differ in the placement of dots(s) or nuqta(s)", e.g. the letter ع ain, with the addition of a nuqta, becomes the letter غ g͟hain.,
Examples from Devanāgari, the script used to write Hindi, are: क़ qa, ख़ kḫa, ग़ ġa, ज़ za, ड़ da, ढ़ ṛha, फ़ fa, झ़ zha, modifying क ka, ख kha, ग ga, ज ja, ड ḍa, ढ ḍha, फ pha, झ jha, respectively. These phonemes have marginal existence in Hindi, occurring in some Perso-Arabic loanwords. The term nuqtā नुक़्ता itself is an example; other examples include क़िला (قلعہ) qilā "fortress", and आग़ा ख़ान Āgā Khān (آغا خان, combination of a Perso-Arabic (aga) and a Turko-Mongolic (khan) honorific, now the title of the leader of the Nizari Ismaili sect.
The nuqtā, and the phonological distinction it represents, is sometimes ignored in practice, i.e. क़िला qilā can simply be spelled as किला kilā. Manisha Kulshreshtha and Ramkumar Mathur write in the text Dialect Accent Features for Establishing Speaker Identity that "A few sounds, borrowed from the other languages like Persian and Arabic, are written with a dot (bindu or nuktā). Many native Hindi speakers, especially those who come from rural backgrounds and do not speak really good Khariboli, pronounce these sounds as the nearest equivalents." For example, these rural speakers will assimilate the sound ɣ (ग़ ) as ɡ (ग ). However, a text on modern Hindi grammar by one author Vajpeyi (1957ff.) allows for the nuqtā in only two letters, ड़ ṛa and ढ़ ṛha, arguing that the other letters written with nuqtā show no phonological differentiation in spoken Hindi, so that writing the nuqtā would be just a pedantic exercise in orthography, or etymology. With these differing recommendations, "there is no uniformity among the Hindi users in the use of these adapted consonants."
With a renewed Hindi-Urdu language contact, many Urdu writers now publish their works in Devanagari editions. Since the Perso-Arabic orthography is preserved in Nastaʿlīq script Urdu orthography, these writers use the nuqtā in Devanāgari when transcribing these consonants.