From The Art and Popular Culture Encyclopedia
A lingua franca is any language widely used beyond the population of its native speakers. The de facto status of lingua franca is usually "awarded" by the masses to the language of the most influential nation(s) of the time. Any given language normally becomes a lingua franca primarily by being used for international commerce, but can be accepted in other cultural exchanges, especially diplomacy. Occasionally the term "lingua franca" is applied to a fully established formal language; thus formerly it was said that French was the lingua franca of diplomacy.
A synonym for lingua franca is “vehicular language.” Whereas a vernacular language is used as a native language in a single speaker community, a vehicular language goes beyond the boundaries of its original community, and is used as a second language for communication between communities. For example, English is a vernacular in England, but is used as a vehicular language (that is, a lingua franca), say, in India.