Along with the English language, Hindi written in the Devanagari script is the official language of the Government of India.  On 14 September 1949, the Constituent Assembly of India adopted Hindi written in Devanagari script as the official language of the Republic of India. To this end, several stalwarts rallied and lobbied pan-India in favor of Hindi, most notably Beohar Rajendra Simha along with Hazari Prasad Dwivedi, Kaka Kalelkar, Maithili Sharan Gupt and Seth Govind Das who even debated in Parliament on this issue. As such, on the 50th birthday of Beohar Rajendra Simha on 14 September 1949, the efforts came to fruition following adoption of Hindi as the official language.  It is one of the 22 scheduled languages of the Republic of India.  However, it is not yet the national language of India because it was not prescribed as such in the Indian constitution.  
Hindi is the lingua franca of the so-called Hindi belt in India. Outside India, it is an official language which is known as Fiji Hindi in Fiji, and is a recognised regional language in Mauritius, Trinidad and Tobago, Guyana, and Suriname.    Apart from specialized vocabulary, Hindi is mutually intelligible with Standard Urdu, another recognized register of Hindustani.
Individually, as a linguistic variety, Hindi is the fourth most-spoken first language in the world, after Mandarin, Spanish and English.  Alongside Urdu as Hindustani, it is the third most-spoken language in the world, after Mandarin and English. 
Part XVII of the Indian Constitution deals with the official language of the Indian Commonwealth. Under Article 343, the official languages of the Union has been prescribed, which includes Hindi in Devanagari script and English:
(1) The official language of the Union shall be Hindi in Devanagari script. The form of numerals to be used for the official purposes of the Union shall be the international form of Indian numerals.  (2) Notwithstanding anything in clause (1), for a period of fifteen years from the commencement of this Constitution, the English language shall continue to be used for all the official purposes of the Union for which it was being used immediately before such commencement: Provided that the President may, during the said period, by order authorize the use of the Hindi language in addition to the English language and of the Devanagari form of numerals in addition to the international form of Indian numerals for any of the official purposes of the Union 
Article 351 of the Indian constitution states
It shall be the duty of the Union to promote the spread of the Hindi language, to develop it so that it may serve as a medium of expression for all the elements of the composite culture of India and to secure its enrichment by assimilating without interfering with its genius, the forms, style and expressions used in Hindustani and in the other languages of India specified in the Eighth Schedule, and by drawing, wherever necessary or desirable, for its vocabulary, primarily on Sanskrit and secondarily on other languages. 
It was envisioned that Hindi would become the sole working language of the Union Government by 1965 (per directives in Article 344 (2) and Article 351),  with state governments being free to function in the language of their own choice. However, widespread resistance to the imposition of Hindi on non-native speakers, especially in South India (such as the those in Tamil Nadu) led to the passage of the Official Languages Act of 1963, which provided for the continued use of English indefinitely for all official purposes, although the constitutional directive for the Union Government to encourage the spread of Hindi was retained and has strongly influenced its policies. 
At the state level, Hindi is the official language of the following Indian states: Bihar, Chhattisgarh, Haryana, Himachal Pradesh, Jharkhand, Madhya Pradesh, Rajasthan, Uttar Pradesh, and Uttarakhand. Each may also designate a "co-official language"; in Uttar Pradesh, for instance, depending on the political formation in power, this language is generally Urdu. Similarly, Hindi is accorded the status of official language in the following Union Territories: Andaman & Nicobar Islands, Chandigarh, Dadra & Nagar Haveli, Daman & Diu, National Capital Territory.
National language status for Hindi is a long-debated theme. In 2010, the Gujarat High Court clarified that Hindi is not the national language of India because the constitution does not mention it as such.   
Outside Asia, the Awadhi language (A Hindi dialect)  is an official language in Fiji as per the 1997 Constitution of Fiji,  where it referred to it as "Hindustani", however in the 2013 Constitution of Fiji, it is simply called "Hindi".  It is spoken by 380,000 people in Fiji. 
Hindi is also spoken by a large population of Madheshis (people having roots in north-India but have migrated to Nepal over hundreds of years) of Nepal. Apart from specialized vocabulary, Hindi is mutually intelligible with Standard Urdu, another recognized register of Hindustani. Hindi is quite easy to understand for some Pakistanis, who speak Urdu, which, like Hindi, is part of Hindustani. Apart from this, Hindi is spoken by the large Indian diaspora which hails from, or has its origin from the " Hindi Belt " of India. A substantially large North Indian diaspora lives in countries like The United States of America, the United Kingdom, the United Arab Emirates, Trinidad and Tobago, Guyana, Suriname, South Africa, Fiji and Mauritius, where it is natively spoken at home and among their own Hindustani-speaking communities. Outside India, Hindi speakers are 8 million in Nepal; 649,000 in United States of America;  450,170 in Mauritius; 380,000 in Fiji;  250,292 in South Africa; 150,000 in Suriname; 100,000 in Uganda; 45,800 in United Kingdom;  20,000 in New Zealand; 20,000 in Germany; 16,000 in Trinidad and Tobago; 3,000 in Singapore.
Like other Indo-Aryan languages, Hindi is considered to be a direct descendant of an early form of Sanskrit, through Sauraseni Prakrit and Śauraseni Apabhraṃśa. Hindi emerged as Apabhramsha (Sanskrit: अपभ्रंश; corruption or corrupted speech), a vernacular form of Prakrit, in the 7th century A.D. 
Standard Hindi is based on the Khariboli dialect,  the vernacular of Delhi and the surrounding region, which came to replace earlier prestige dialects such as Awadhi, Maithili (sometimes regarded as separate from the Hindi dialect continuum) and Braj. Urdu – another form of Hindustani – acquired linguistic prestige in the later Mughal period (1800s), and underwent significant Persian influence. In the late 19th century, a movement to develop Hindi as a standardised form of Hindustani separate from Urdu took form. In 1881, Bihar accepted Hindi as its sole official language, replacing Urdu, and thus became the first state of India to adopt Hindi.
After independence, the government of India instituted the following conventions:
- standardisation of grammar: In 1954, the Government of India set up a committee to prepare a grammar of Hindi; The committee's report was released in 1958 as A Basic Grammar of Modern Hindi.
- standardisation of the orthography, using the Devanagari script, by the Central Hindi Directorate of the Ministry of Education and Culture to bring about uniformity in writing, to improve the shape of some Devanagari characters, and introducing diacritics to express sounds from other languages.
Comparison with Modern Standard Urdu
Linguistically, Hindi and Urdu are two registers of the same language.  Hindi is written in the Devanagari script and uses more Sanskrit words, whereas Urdu is written in the Perso-Arabic script and uses more Arabic and Persian words. Hindi is the most commonly used official language in India. Urdu is the national language and lingua franca of Pakistan and is one of 22 official languages of India.
Traditionally, Hindi words are divided into five principal categories according to their etymology:
- Tatsam (तत्सम "same as that") words: These are words which are spelled the same in Hindi as in Sanskrit (except for the absence of final case inflections). They include words inherited from Sanskrit via Prakrit which have survived without modification (e.g. Hindi नाम nām / Sanskrit नाम nāma, "name"; Hindi कर्म karm / Sanskrit कर्म karma, "deed, action; karma "), as well as forms borrowed directly from Sanskrit in more modern times (e.g. प्रार्थना prārthanā, "prayer"). Pronunciation, however, conforms to Hindi norms and may differ from that of classical Sanskrit. Amongst nouns, the tatsam word could be the Sanskrit non-inflected word-stem, or it could be the nominative singular form in the Sanskrit nominal declension.
- Ardhatatsam (अर्धतत्सम "semi-tatsama") words: Such words are typically earlier loanwords from Sanskrit which have undergone sound changes subsequent to being borrowed. (e.g. Hindi सूरज sūraj from Sanskrit सूर्य surya)
- Tadbhav (तद्भव "born of that") words: These are native Hindi words derived from Sanskrit after undergoing phonological rules (e.g. Sanskrit कर्म karma, "deed" becomes Sauraseni Prakrit कम्म kamma, and eventually Hindi काम kām, "work") and are spelled differently from Sanskrit.
- Deshaj (देशज) words: These are words that were not borrowings but do not derive from attested Indo-Aryan words either. Belonging to this category are onomatopoetic words or ones borrowed from local non- Indo-Aryan languages.
- Videshī (विदेशी "foreign") words: These include all loanwords from non-indigenous languages. The most frequent source languages in this category are Persian, Arabic, English and Portuguese. Examples are कमेटी kameṭī from English committee and साबुन sābun "soap" from Arabic.
Much of Modern Standard Hindi's vocabulary is derived from Sanskrit, either as native tadbhav words or tatsam borrowings from Sanskrit, especially in technical and academic fields. The Hindi standard, from which much of the Persian, Arabic and English vocabulary has been replaced by neologisms compounding tatsam words, is called Shuddh Hindi (pure Hindi), and is viewed as a more prestigious dialect over other more colloquial forms of Hindi.
Excessive use of tatsam words creates problems for native speakers. They may have Sanskrit consonant clusters which do not exist in native Hindi. The educated class of India may be able to pronounce such words, but others have difficulty.
Arabic also shows influence in Hindi, often via Persian but sometimes directly.
Hindi literature is broadly divided into four prominent forms or styles, being Bhakti (devotional – Kabir, Raskhan); Shringar (beauty – Keshav, Bihari); Virgatha (extolling brave warriors); and Adhunik (modern).
Medieval Hindi literature is marked by the influence of Bhakti movement and the composition of long, epic poems. It was primarily written in other varieties of Hindi, particularly Avadhi and Braj Bhasha, but also in Khariboli. During the British Raj, Hindustani became the prestige dialect. Hindustani with heavily Sanskritised vocabulary or Sahityik Hindi (Literary Hindi) was popularised by the writings of Swami Dayananda Saraswati, Bhartendu Harishchandra and others. The rising numbers of newspapers and magazines made Hindustani popular with the educated people.
Chandrakanta, written by Devaki Nandan Khatri in 1888, is considered the first authentic work of prose in modern Hindi.  The person who brought realism in the Hindi prose literature was Munshi Premchand, who is considered as the most revered figure in the world of Hindi fiction and progressive movement.
The Dwivedi Yug ("Age of Dwivedi") in Hindi literature lasted from 1900 to 1918. It is named after Mahavir Prasad Dwivedi, who played a major role in establishing the Modern Hindi language in poetry and broadening the acceptable subjects of Hindi poetry from the traditional ones of religion and romantic love.
In the 20th century, Hindi literature saw a romantic upsurge. This is known as Chhayavaad (shadowism) and the literary figures belonging to this school are known as Chhayavaadi. Jaishankar Prasad, Suryakant Tripathi 'Nirala', Mahadevi Varma and Sumitranandan Pant, are the four major Chhayavaadi poets.
Uttar Adhunik is the post-modernist period of Hindi literature, marked by a questioning of early trends that copied the West as well as the excessive ornamentation of the Chhayavaadi movement, and by a return to simple language and natural themes.
The following is a sample text in High Hindi, of the Article 1 of the Universal Declaration of Human Rights (by the United Nations):
- अनुच्छेद 1 (एक) – सभी मनुष्यों को गौरव और अधिकारों के विषय में जन्मजात स्वतन्त्रता और समानता प्राप्त हैं। उन्हें बुद्धि और अन्तरात्मा की देन प्राप्त है और परस्पर उन्हें भाईचारे के भाव से बर्ताव करना चाहिए।
- Transliteration (IAST)
- Anucched 1 (ek) – Sabhī manuṣyõ ko gaurav aur adhikārõ ke viṣay mẽ janmajāt svatantratā aur samāntā prāpt hai. Unhẽ buddhi aur antarātmā kī den prāpt hai aur paraspar unhẽ bhāīcāre ke bhāv se bartāv karnā cāhie.
- Transcription (IPA)
- [ənʊtʃʰːeːd̪ eːk | səbʱiː mənʊʃjõː koː ɡɔːɾəʋ ɔːr əd̪ʱɪkaːɾõ keː maːmleː mẽː dʒənmədʒaːt̪ sʋət̪ənt̪ɾət̪aː ɔːr səmaːntaː pɾaːpt̪ hɛː ‖ ʊnʱẽ bʊd̪ʱːɪ ɔːɾ ənt̪əɾaːt̪maː kiː d̪eːn pɾaːpt̪ hɛː ɔːɾ pəɾəspəɾ ʊnʱẽː bʱaːiːtʃaːɾeː keː bʱaːʋ seː bəɾt̪aːʋ kəɾnə tʃaːhɪeː ‖]
- Gloss (word-to-word)
- Article 1 (one) – All human-beings to dignity and rights' matter in from-birth freedom and equality acquired is. Them to reason and conscience's endowment acquired is and always them to brotherhood's spirit with behaviour to do should.
- Translation (grammatical)
- Article 1 – All human beings are born free and equal in dignity and rights. They are endowed with reason and conscience and should act towards one another in a spirit of brotherhood.
- Anti-Hindi agitations of Tamil Nadu
- Bengali Language Movement (Manbhum)
- Hindi Divas – the official day to celebrate Hindi as a language.
- Languages of India and Languages with official status in India
- List of English words of Hindi or Urdu origin
- list of Hindi words at Wiktionary, the free dictionary
- List of languages by number of native speakers in India
- List of Sanskrit and Persian roots in Hindi
- Stop Hindi Imposition Campaign
- World Hindi Secretariat