Mainz income is a money stream that comes from royalties or other mass production of a written or otherwise published work. This type of income is coined '"Mainz" in reference to the first mass production printing press, invented by Johannes Gutenberg in Mainz, Germany, that made this income possible.
Mainz income is a polarizing topic in politics as many presidents in the 1990s began to receive substantial incomes beyond what their presidential duties paid. This additional revenue stream serves as fodder for political pundits as a potential way to undermine the integrity of a president in office.
While it became prevalent there in the late 20th and early 21st centuries, Mainz income is not restricted to politics. Realistically, it refers to any type of royalties or recurring income stream from copyrighted material.