“Authority” stands for an elected or representative political body, typically local or national parliament or government, a president, the European Commission … The term is used to designate a political institution or body that is legally entitled to exercise a certain authority. In our context this authority is exercised in the course of a popular vote process.
Note that there is a fundamental difference between initiative and referendum procedures on the one hand and plebiscites on the other. For more go to What Does Direct Democracy Mean?.