An ELT Glossary : Lingua Franca
A lingua franca is the language used by a group of people who do not all share a common language but have to communicate in speech and/or writing. For example, a Swedish tourist in France who didn't speak French might use English to check into their hotel, ask for travel directions, etc. Similarly, in a business meeting in a multinational company, where there are speakers from four or five different language groups, one language will be chosen as the language of communication between them. This may be the language of the parent company or simply a commonly spoken language like English, depending on the company policy.
In large organisations like the European Union, members speak a wide variety of firstlanguages and the possibility of using specific languages has to be formalised - at the time of writing the EU has 24 official languages. In high level meetings, participants are free to use any of the official languages, with a team of simultaneous interpreters translating what they say into the other 23. At lower level meetings, either English or French are currently the languages most commonly chosen as the lingua franca but, again, potentially any of the official languages could be chosen.