A “lingua franca” (line 1) is a language that:
has been chosen to be used in one specific field for communication, such as English chosen to be used in research.
has more visibility and widespread use due to its simple and easy-to-learn features, such as those in English.
that was created in order to facilitate communication between speakers of different languages, just like Esperanto.
is used for mutual communication between speakers of different languages, even if none of them speaks the lingua franca.