A lingua franca (/ˌlɪŋɡwə ˈfræŋkə/ ( listen); lit. Frankish tongue), also known as a bridge language, common language, trade language, auxiliary language, vehicular language, or link language is a language or dialect systematically used to make communication possible between people who do not share a native language or dialect, particularly when it is a third language that is distinct from both of the speakers' native languages.
Lingua francas have developed around the world throughout human history, sometimes for commercial reasons (so-called "trade languages" facilitated trade) but also for cultural, religious, diplomatic and administrative convenience, and as a means of exchanging information between scientists and other scholars of different nationalities. The term is taken from the medieval Mediterranean Lingua Franca, a Romance-based pidgin language used by European merchants and sailors during the 2nd millennium. A world language – a language spoken internationally and learned and spoken by a large number of people – is a language that may function as a global lingua franca.
Lingua Franca refers to any language used for communication between people who do not share a native language. It can refer to hybrid languages such as pidgins and creoles used for communication between language groups. It can also refer to languages which are native to one nation (often a colonial power) but used as a second language for communication between groups. Finally, lingua franca can refer to a third language which allows inter-comprehension among people speaking different mother tongues as a neutral language or jargon of which nobody can claim ownership. Lingua Franca is a functional term, independent of any linguistic history or language structure.
Whereas a vernacular language is the native language of a specific geographical community, a lingua franca is used beyond the boundaries of its original community, for trade, religious, political or academic reasons. For example, English is a vernacular in the United Kingdom but is used as a lingua franca in the Philippines. Arabic, French, Mandarin Chinese, Spanish, Portuguese, and Russian, serve a similar purpose as industrial/educational lingua francas, across regional and national boundaries.
The term lingua franca derives from Mediterranean Lingua Franca, the "language of the Francs", "Francs" being the term used to refer to all Western Europeans as opposed to the Greeks — is considered to be a translation of the Arab lûghat al-Ifranj, which appears for the first time in an Arabic text from the 9th century. However, Western speakers and their contemporaries only began to use the term "lingua franca" to describe this phenomenon at the beginning of the 16th century and the language that people around the Levant and the eastern Mediterranean Sea used as the main language of commerce and diplomacy from late medieval times, especially during the Renaissance era, to the 18th century and allowed communication between the people living, fighting in the area without becoming the vernacular language of any of them.  At that time, Italian-speakers dominated seaborne commerce in the port cities of the Ottoman Empire and a simplified version of Italian, including many loan words from Greek, Old French, Portuguese, Occitan, and Spanish as well as Arabic and Turkish came to be widely used as the "lingua franca" (in the generic sense used) of the region.
In Lingua Franca (the specific language), lingua means a language, as in Portuguese and Italian, and franca is related to phrankoi in Greek and faranji in Arabic as well as the equivalent Italian. In all three cases, the literal sense is "Frankish", but the name actually applied to all Western Europeans during the late Byzantine Empire.
The Douglas Harper Etymology Dictionary states that the term Lingua Franca (as the name of the particular language) was first recorded in English during the 1670s, although an even earlier example of the use of Lingua Franca in English is attested from 1632, where it is also referred to as "Bastard Spanish".
As recently as the late 20th century, some restricted the use of the generic term to mean only hybrid languages that are used as vehicular languages, its original meaning, but it now refers to any vehicular language.
The term is well established in its naturalization to English, which is why major dictionaries do not italicize it as a "foreign" term. Its plurals in English are lingua francas and linguae francae, with the first of those being first-listed or only-listed in major dictionaries.
The use of lingua francas has existed since antiquity. The first example of lingua franca in the ancient world, before Greek and Latin, was Aramaic. Latin and Koine Greek were the lingua francas of the Roman Empire and the Hellenistic culture. Akkadian (died out during Classical antiquity) and then Aramaic remained the common languages of a large part of Western Asia from several earlier empires.
In certain countries, the lingua franca is also the national language. Indonesian has the same function in Indonesia, although Javanese has more native speakers. Still, Indonesian is the sole official language and is spoken throughout the country. Also Persian is both the lingua franca of Iran and its national language.
The Hindustani language (Hindi-Urdu) is the lingua franca of Pakistan and Northern India. Many Indian states have adopted the Three-language formula in which students in Hindi speaking states are taught: "(a) Hindi (with Sanskrit as part of the composite course); (b) Urdu or any other modern Indian language and (c) English or any other modern European language." The order in non-Hindi speaking states is: "(a) the regional language; (b) Hindi; (c) Urdu or any other modern Indian language excluding (a) and (b); and (d) English or any other modern European language." Hindi has also emerged as a lingua franca for the locals of Arunachal Pradesh, a linguistically diverse state in Northeast India.It is estimated that 90 percent of the state's population knows Hindi.
The only documented sign language used as a lingua franca is Plains Indian Sign Language, used across much of North America. It was used as a second language across many indigenous peoples. Alongside or a derivation of Plains Indian Sign Language was Plateau Sign Language, now extinct. Inuit Sign Language could be a similar case in the Arctic among the Inuit for communication across oral language boundaries, but little research exists.
Franc and (prefix) franco- (Φράγκος Phrankos and φράγκο- phranko-
... Hindustani is the lingua franca of both India and Pakistan ...
... By the time of British colonialism, Hindustani was the lingua franca of all of northern India and what is today Pakistan ...