Plebiscito vs. referendo
In Portuguese, both referendo and plebiscito refer to direct popular votes where citizens decide on a specific issue. However, they are not interchangeable. The crucial difference between them lies in the timing of the vote in relation to the creation of a law or government act.
Plebiscito
B2A plebiscito is a popular consultation that happens before a law or administrative act is created. The government asks the population for their opinion on a fundamental political or social matter. The outcome of the vote then serves as a guideline for the legislative body (like Congress) to create the corresponding law or take action.
Em 1993, o Brasil teve um plebiscito para escolher o sistema de governo.
(In 1993, Brazil held a plebiscite to choose the system of government.)
Foi proposto um plebiscito sobre a possível divisão do estado em três novos territórios.
(A plebiscite was proposed regarding the possible division of the state into three new territories.)
O povo votará em um plebiscito para decidir se o país deve adotar o parlamentarismo.
(The people will vote in a plebiscite to decide if the country should adopt parliamentarism.)
Antes de tomar a decisão, o governo realizará um plebiscito para consultar a vontade popular.
(Before making the decision, the government will hold a plebiscite to consult the popular will.)
Referendo
B2A referendo is a popular vote that happens after a law has already been passed by the legislative body. The population is then called to either ratify (approve) or reject the specific piece of legislation that already exists. In essence, the people are given the final say to validate or veto a law made by their representatives.
O referendo de 2005 no Brasil foi sobre um artigo da lei que proibia o comércio de armas.
(The 2005 referendum in Brazil was about an article of the law that prohibited the sale of firearms.)
A nova lei tributária foi aprovada pelo Congresso, mas precisa ser confirmada em um referendo.
(The new tax law was approved by Congress, but it needs to be confirmed in a referendum.)
A população rejeitou a emenda constitucional no referendo do último domingo.
(The population rejected the constitutional amendment in last Sunday's referendum.)
Para que o tratado internacional entre em vigor, ele deve ser submetido a um referendo.
(For the international treaty to come into force, it must be submitted to a referendum.)
Summary
In summary, the core distinction is the timing. A plebiscito is a vote on an issue before the law is written, essentially asking the people, What should we do?. A referendo is a vote to approve or reject a law that has already been created by the legislature, effectively asking, Do you agree with this law we have made?.