a person whose material wealth is valued at more than a million dollars
"After selling his tech startup, he became a millionaire overnight."
A person whose net worth is at or greater than one million units of the local currency, but less than two million.
"After selling his small tech startup, he became a millionaire in Canadian dollars but still has not reached two million yet."
In plain English: A millionaire is someone who has at least one million dollars worth of money and assets combined.
"He was invited to speak at the charity gala because he is a millionaire who donates most of his fortune to education."
Usage: Use this term to describe anyone with assets between $1 and $2 million in a specific currency. Avoid applying it loosely to those who merely earn high incomes without possessing that level of net worth.
The word millionaire comes from French and originally meant a person who owns one million units of currency. It entered English through direct borrowing rather than being formed by combining the existing English words "million" and "-aire."