origin: (1620s) Italian; Lingua Franca= Frankish tongue.
The opposite of a vernacular; if two people meet each other and speak different languages, then how might they communicate? Often it is through a common tongue or second language that they both share (even if in limited amounts such as "pidgin"). Internationally speaking, English and French bridge a lot of gaps. Famously, Esperanto tried -- and failed -- to bridge the gaps of all languages. Alas, we still suffer from Babylon.
However, the term can also refer to conceptual ideas or context shared, such as Germany's classic from the 1920's, The Cabinet of Dr. Caligari being a "lingua franca" -- a thematic communication device -- for all modern horror and suspense films to follow.
The term refers to a language used in the Levant (Eastern Mediterranean), bare-bones Italian mixed together with bits of Spanish, French, Turkish, Greek, and Arabic. During the Crusades, Arabs referred to all Europeans as "Franks" and the term may stem from there. The term "Bastard Spanish" implies the same as Lingua Franca.