Refers to an extremely wealthy person, often with a nuance of vast, almost unimaginable riches. The term is somewhat literary or emphatic compared to more common words like 大富豪.
He became a multimillionaire in one generation.
Multimillionaires have built villas on that island.
大富豪 is a more common, neutral term for a very wealthy person, while 千万長者 carries a slightly more emphatic or literary feel.
億万長者 specifically implies a billionaire (hundreds of millions), whereas 千万長者 suggests tens of millions, though both are often used loosely for extreme wealth.
Compound of 千万 (senban, 'ten million; countless') and 長者 (chōja, 'wealthy person; elder'). The exact historical derivation is uncertain, but the term evokes a person of immense, almost incalculable wealth.