lingua franca
Definition - A language that has been adopted as a common language by people who don't speak the same language.

Example -
Medieval Latin was Europe's lingua franca during the Middle Ages.

Etymology -
In Italian the term means "Frankish tongue."
Note: The first lingua franca was a minimalist version of Italian — mixed with Spanish, French, Turkish, Arabic, and Greek words — that was used by Levantine traders 500 years ago. Because the Arabs used to call all Europeans Franks, their language was called the Frankish tongue.

Oxford English Dictionary -
The phrase's first citation is from 1678:
"'Tis a kind of Lingua Franca, as I have heard the Merchants call it; a certain compound Language, made up of all Tongues, that passes through the Levant."
(Dryden Limberham i. i)

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