A lingua franca (; lit. 'Frankish tongue'; for plurals see § Usage notes), 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 groups of 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, an Italian-based pidgin language used especially by traders as a lingua franca in the Mediterranean Basin from the 11th to the 19th century.  A world language - a language spoken internationally and by many people - is a language that may function as a global lingua franca.
A lingua franca is any language used for communication between people who do not share a native language. It can refer to mixed 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 diverse language communities in a colony or former colony. Lingua franca is a functional term, independent of any linguistic history or language structure.
Lingua francas are often pre-existing languages with native speakers, but they can also be pidgin or creole languages developed for that specific region or context. Pidgin languages are rapidly developed and simplified combinations of two or more established languages, while creoles are generally viewed as pidgins that have evolved into fully complex languages in the course of adaptation by subsequent generations. Pre-existing lingua francas such as French are used to facilitate intercommunication in large-scale trade or political matters, while pidgins and creoles often arise out of colonial situations and a specific need for communication between colonists and indigenous peoples. Pre-existing lingua francas are generally widespread, highly developed languages with many native speakers. Conversely, pidgin languages are very simplified means of communication, containing loose structuring, few grammatical rules, and possessing few or no native speakers. Creole languages are more developed than their ancestral pidgins, utilizing more complex structure, grammar, and vocabulary, as well as having substantial communities of native speakers. 
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 it is used as a lingua franca in the Philippines, alongside Filipino. Arabic, French, Mandarin Chinese, Spanish, Portuguese, Hindustani, 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 pidgin 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.At that time, a simplified version of mainly Italian on the east and Spanish on the west side of the Mediterranean including many loan words from Greek, the Slavic languages, Arabic, and Turkish came to be widely used as the "lingua franca" of the region, although some scholars claim that the Mediterranean Lingua Franca was just poorly used Italian. 
In Lingua Franca (the specific language), lingua means a language, as in 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", leading to the direct translation: "language of the Franks". During the late Byzantine Empire, "Franks" was a term that applied to all Western Europeans.
Through changes of the term in literature, Lingua Franca has come to be interpreted as a general term for pidgins, creoles, and some or all forms of vehicular languages. This transition in meaning has been attributed to the idea that pidgin languages only became widely known from the 16th century on due to European colonization of continents such as The Americas, Africa, and Asia. During this time, the need for a term to address these pidgin languages arose, hence the shift in the meaning of Lingua Franca from a single proper noun to a common noun encompassing a large class of pidgin languages.
Douglas Harper's Online 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".
The use of lingua francas has existed since antiquity. 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.
The majority of pre-colonial North American nations communicated internationally using Hand Talk. Also called Prairie Sign Language,Plains Indian Sign Language, or First Nations Sign Language, this language functioned predominately--and still contintues to function--as a second language within most of the (now historical) countries of the Great Plains, from Newe Segobia in the West to Anishinaabewaki in the East, down into what are now the northern states of Mexico and up into Cree Country stopping before Denendeh. It remains unknown the relationship between Hand Talk and other manual Indigenous languages like Keresan Sign Language and Plateau Sign Language, the latter of which has gone to sleep (though Ktunaxa Sign Language is still spoken). Although unrelated, perhaps Inuit Sign Language played and continues to play a similar role across Inuit Nunangat and the various Inuit dialects. The original Hand Talk is found across Indian Country in pockets, but it has also been employed to create new or revive old languages, such as with Oneida Sign Language.
Sogdian was used to facilitate trade between those who spoke different languages along the Silk Road, which is why native speakers of Sogdian were employed as translators in Tang China.  The Sogdians also ended up circulating spiritual beliefs and texts, including those of Buddhism and Christianity, thanks to their ability to communicate to many people in the region through their native language. 
The Hindustani language (Hindi-Urdu) is the lingua franca of Pakistan and Northern India.[self-published source?][page needed] 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.
Indonesian - which originated from a Malay language variant spoken in Riau - is the official language and a lingua franca in Indonesia and widely understood across the Malay world including Malaysia, Singapore and Brunei, although Javanese has more native speakers. Still, Indonesian is the sole official language and is spoken throughout the country even though it is the first language of a very small minority of Indonesians. 
Swahili developed as a lingua franca between several Bantu-speaking tribal groups on the east coast of Africa with heavy influence from Arabic. The earliest examples of writing in Swahili are from 1711. In the early 19th century the use of Swahili as a lingua franca moved inland with the Arabic ivory and slave traders. It was eventually adopted by Europeans as well during periods of colonization in the area. German colonizers used it as the language of administration in German East Africa, later becoming Tanganyika, which influenced the choice to use it as a national language in what is now independent Tanzania. Swahili (known to natives as Kiswahili) is currently one of the national languages and it is taught in schools and universities in several East African countries, thus prompting it to be regarded as a modern-day lingua franca by many people in the region. Several Pan-African writers and politicians have unsuccessfully called for Swahili to become the lingua franca of Africa as a means of unifying the African continent and overcoming the legacy of colonialism. 
English is often used by non-native English speakers who have different mother tongues to communicate with one another. In the European Union, the use of English as a lingua franca has led researchers to investigate whether a new dialect of English (Euro English) has emerged.
When the United Kingdom became a colonial power, English served as the lingua franca of the colonies of the British Empire. In the post-colonial period, some of the newly created nations which had multiple indigenous languages opted to continue using English as one of their official languages. A couple of examples of these nations are Ghana and South Africa. 
Although not spoken as a first language by most African French speakers, French is a lingua franca in most Western and Central African countries and an official language of many, a remnant of French and Belgian colonialism. These African countries and others are members of the Francophonie. 
International Sign, though a pidgin language, is present at most significant international gatherings, from which interpretations of national sign languages are given, such as in LSF, ASL, BSL, Libras, or Auslan. International Sign, or IS and formerly Gestuno, interpreters can be found at many European Union parliamentary or committee sittings, during certain United Nations affaires, conducting international sporting events like the Deaflympics, in all World Federation of the Deaf functions, and across similar settings. The language has few set internal grammatical rules, instead co-opting national vocabularies of the speaker and audience, and modifying the words to bridge linguistic gaps, with heavy use of gesture and classifiers.
Russian is in use and widely understood in Central Asia and the Caucasus, areas formerly part of the Russian Empire and Soviet Union, and in much of Central and Eastern Europe. It remains the official language of the Commonwealth of Independent States. Russian is also one of the six official languages of the United Nations.
Persian, an Iranian language, is the official language of Iran, Afghanistan (Dari) and Tajikistan (Tajik). It acts as a lingua franca in both Iran and Afghanistan between the various ethnic groups in those countries. The Persian language in South Asia, before the British colonized the Indian subcontinent, was the region's lingua franca and a widely used official language in north India and Pakistan.
Old Church Slavonic, an Eastern South Slavic language, is the first Slavic literary language. Between 9th and 11th century it was lingua franca of a great part of the predominantly Slavic states and populations in Southeast and Eastern Europe, in liturgy and church organization, culture, literature, education and diplomacy. 
Hausa can also be seen as a lingua franca because it is the language of communication between speakers of different languages in Northern Nigeria and other West African countries , including the northern region of Ghana. 
In Djibouti and parts of Eritrea, both countries where multiple official languages are spoken, Arabic has emerged as a lingua franca in part thanks to the population of the region being predominantly Muslim and Arabic playing a crucial role in the religion of Islam. In addition, after having fled from Eritrea due to ongoing warfare and gone to some of the nearby Arab countries, Eritrean emigrants are contributing to Arabic becoming a lingua franca in the region by coming back to their homelands having picked up the Arabic language. 
In Qatar, the medical community is primarily made up of workers from countries without English as a native language. In medical practices and hospitals, nurses typically communicate with other professionals in English as a lingua franca. This occurrence has led to interest in researching the consequences and affordances of the medical community communicating in a lingua franca.
Up to now [constructed languages] have all proved transient and none has actually achieved the status of lingua franca with a large community of fluent speakers.
Franc and (prefix) franco- (? Phrankos and - phranko-)
... Hindustani is the lingua franca of both India and Pakistan ...[self-published source]
... By the time of British colonialism, Hindustani was the lingua franca of all of northern India and what is today Pakistan ...