A lingua franca (/ˌlɪŋɡwə ˈfræŋkə/), also known as a bridge language, common language, trade language or vehicular language, is a language or dialect systematically (as opposed to occasionally, or casually) 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 native languages.
Lingua francas have developed around the world throughout human history, sometimes for commercial reasons (so-called "trade languages") 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 originates with one such language, Mediterranean Lingua Franca.
Lingua franca is a term defined functionally, that is "independently of the linguistic history or structure of the language". Pidgins and creoles often function as lingua francas, but many such languages are neither pidgins nor creoles.
Whereas a vernacular language is used as a native language in a community, a lingua franca is used beyond the boundaries of its original community and is used as a second language for communication between groups. For example, English is a vernacular in the United Kingdom but is used as a lingua franca in the Philippines and India. Russian and French serve a similar purpose as industrial/educational lingua francas in many areas.
International auxiliary languages such as Esperanto have not had a great degree of adoption globally so they cannot be described as global lingua francas.
The term lingua franca originated as the name of a particular language that was used around the eastern Mediterranean Sea as the main language of commerce and diplomacy, from late medieval times, especially during the Renaissance era, to the 18th century. 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 was 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, the use of the generic term was restricted by some to mean only hybrid languages that are used as vehicular languages, its original meaning, but it now refers to any vehicular language.
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 and then Aramaic remained the common languages of a large part of Western Asia from several earlier empires. Examples of lingua francas remain numerous and exist on every continent. The most obvious example as of the early 21st century is English. There are many other lingua francas in particular regions, such as French, Spanish, Urdu, Hindi, Portuguese, Russian, Arabic, Mandarin, and Swahili.
In certain countries, the lingua franca is also the national language. Urdu is the lingua franca of Pakistan as well as the national language.
Indonesian has the same function in Indonesia, but Javanese has more native speakers. Still, Indonesian is the sole official language and is spoken, often as a second language, throughout the country.
Finally, the only documented widespread lingua franca to be a sign language 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.