Translation guide
A person with a net worth of at least one billion units of a currency, typically US dollars. In Japanese, the term is often expressed with 億万長者 (okumanchōja), though this literally means 'multimillionaire' and can be ambiguous. For clarity, ビリオネア (birionea) is used, especially in modern contexts.
Referring to a person whose wealth is in the billions (of dollars, yen, etc.)
Specifically a billionaire in Japanese yen (approximately 10 million USD at 100 yen to the dollar, but the term is used loosely).
億万長者 is the traditional term and can be ambiguous (millionaire or billionaire). ビリオネア is a direct loanword and is unambiguous, but may sound more technical or foreign. In casual conversation, 億万長者 is more common even for billionaires.
Do not translate 'billionaire' as 十億長者 (juuokuchouja) or similar literal constructions; these are not used in Japanese.
The most common Japanese term for an extremely wealthy person, literally 'millionaire of millions'. It can refer to both millionaires and billionaires, so context is needed.
彼は若くして億万長者になった。
He became a billionaire at a young age.
Loanword from English, specifically meaning 'billionaire'. Used in news and business contexts to avoid ambiguity.
世界のビリオネアの数は増え続けている。
The number of billionaires in the world continues to increase.
A person of wealth, a wealthy individual. Not specific to billionaires; can refer to anyone with substantial assets.
彼は有名な資産家だ。
He is a famous wealthy individual.
In Japan, 億万長者 often implies wealth in the hundreds of millions or billions of yen. Since 1億 (oku) is 100 million, 億万 suggests multiples of that.
円で億万長者になるのは大変だ。
It's tough to become a billionaire in yen.