lingua franca
This X when two or more people use the same word X to refer to the same meaning M.
There is also the expression public language, which tends to be associated with a very large set of people. ---
This page is auto-translated from /nishio/共通言語 using DeepL. If you looks something interesting but the auto-translated English is not good enough to understand it, feel free to let me know at @nishio_en. I'm very happy to spread my thought to non-Japanese readers.