The expression 'Lingua franca' means

Lingua franca (noun):
1) A common language consisting of Italian mixed with French, Spanish, Greek, and Arabic that was formerly spoken in Mediterranean ports.
2) Any of various languages used as common or commercial tongues among peoples of diverse speech.
English is used as a lingua franca among many airline pilots.
3) Something resembling a common language.
Movies are the lingua franca of the twentieth century.

Source: Merriam-Webster Dictionary.