language used as a means of communication between populations speaking vernaculars that are not mutually intelligible. The term was first used during the Middle Ages to describe a French- and Italian-based jargon, or pidgin, that was developed by Crusaders and traders in the eastern Mediterranean and characterized by the invariant forms of its nouns, verbs, and adjectives. These changes have been interpreted as simplifications of the Romance languages.

Because they bring together very diverse groups of people, many empires and major trade entrepôts have had lingua francas. If pidgins have sometimes been defined, less informatively, as lingua francas, it is because they evolved from varieties that had served as trade languages. Aramaic (Aramaic language) played this role in Southwest Asia from as early as the 7th century BC to approximately AD 650. Classical Latin (Latin language) was the dominant lingua franca of European scholars until the 18th century, while a less prestigious variety of Latin served as that of the Hanseatic League (13th–15th centuries), especially in its bookkeeping.

During the era of European exploration in the 15th–18th centuries, Portuguese (Portuguese language) served as a diplomatic and trade language in coastal Africa and in Asian coastal areas from the Indian Ocean to Japan. In Southeast Asia, meanwhile, Malay (Malay language) was already serving as an important lingua franca; it had been adopted by Arab and Chinese traders in the region well before the Europeans arrived. Later both the Dutch and the British used Malay for communication with the peoples resident in the region.

Modern lingua francas may or may not be officially designated as such: the United Nations employs six official languages (Arabic (Arabic language), Chinese (Chinese languages), English (English language), French (French language), Russian (Russian language), and Spanish (Spanish language)); international air traffic control (traffic control) uses English as a common language; and some multilingual Asian and African countries have unofficial lingua francas that facilitate interethnic or interregional communication. Such languages may be erstwhile pidgins, as with Lingala (Lingala language) in the Democratic Republic of the Congo, the Nigerian and Cameroon pidgins, or Hiri Motu and Tok Pisin in Papua New Guinea; they may also be non-pidginized varieties such as Swahili (Swahili language) in East Africa or Hausa (Hausa language) in West Africa.