A(n) is the process by which citizens propose legislation or constitutional amendments through the petition process followed by a popular vote. gerrymander initiative mandate referendum

A referendum is the process by which citizens propose legislation or constitutional amendments through the petition process followed by a popular vote. 
An entire electorate (all the people in a country or an area) is invited to vote on a particular proposal. Example: A referendum was held on the question of whether the U.K. should remain a member of the European Community.