Bilionário vs. multimilionário
The difference between bilionário and multimilionário generally depends on the region (Brazil vs. Portugal) and the mathematical scale used. In Brazil, bilionário maps directly to the English "billionaire" ($1,000,000,000). In Portugal, multimilionário is often used to describe this same level of wealth because the European numerical scale defines a bilionário as someone with a million millions (a trillion in English).
Bilionário
B1This is the direct translation of "billionaire" in Brazilian Portuguese, referring to someone with assets worth at least one thousand million ($1,000,000,000). In European Portuguese, bilionário technically refers to a "trillionaire" (10^12), though media influence is slowly changing this usage.
O novo aplicativo tornou o jovem um bilionário da noite para o dia.
(The new app made the young man a billionaire overnight.)
A lista da Forbes classifica a pessoa mais rica como bilionário.
(The Forbes list ranks the richest person as a billionaire.)
Para ser bilionário no Brasil, é preciso ter um patrimônio imenso.
(To be a billionaire in Brazil, one needs to have immense net worth.)
Multimilionário
B2Literally translating to "multi-millionaire", this refers to someone with many millions. In Brazil, this usually implies someone rich who has not yet reached the billion mark. However, in Portugal, this is the standard term used for what Americans call a "billionaire" (1,000,000,000), since that number is mathematically considered "one thousand millions" rather than a full "billion" in Europe.
Ele é um multimilionário, mas ainda não chegou ao primeiro bilhão.
(He is a multimillionaire, but he hasn't reached his first billion yet.)
Em Portugal, o vencedor da loteria foi chamado de multimilionário.
(In Portugal, the lottery winner was called a billionaire (literally: multimillionaire).)
Vários empresários são multimilionários com fortunas de centenas de milhões.
(Several businessmen are multimillionaires with fortunes of hundreds of millions.)
Summary
Use bilionário in Brazil for someone with 1,000,000,000 currency units. Use multimilionário in Portugal for that same amount (since it is viewed as 1,000 millions). In Brazil, multimilionário simply means very wealthy but less than a bilionário.