- a common language used by people of diverse backgrounds to communicate with one another, often a basic form of speech with simplified grammar
- The most common plural form in English is lingua francas; however, the plural forms lingue franche (following the Italian) and linguae francae (in the style of Latin) are both fairly common as well. Also attested, but quite rare, and probably nonstandard, is the form linguas franca (treating lingua alone as the noun and taking franca as an adjective; compare attorneys general, forests primeval).
- (common language): koine
- (common language): vernacular