Etymology
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billionaire (n.)

1844, American English, from billion on model of millionaire. Marked "[Rare.]" in Century Dictionary (1902). The first in the U.S. likely was John D. Rockefeller, some time after World War I.

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Definitions of billionaire

billionaire (n.)
a very rich person whose material wealth is valued at more than a billion dollars;
From wordnet.princeton.edu