lingua franca

Concise Oxford English Dictionary © 2008 Oxford University Press:
lingua franca /ˌlɪŋgwə ˈfraŋkə/

▶noun (pl. lingua francas)

• 1 a language used as a common language between speakers whose native languages are different.

• 2 historical a mixture of Italian with French, Greek, Arabic, and Spanish, formerly used in the eastern Mediterranean.

– origin C17: from Ital., lit. ‘Frankish tongue’.