n. Somebody whose wealth is greater than one billion (109) dollars, or other currency.
The Century Dictionary and Cyclopedia
n. One who possesses property worth a billion reckoned in standard coin of the country.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. a very rich person whose material wealth is valued at more than a billion dollars
Word Usage
"The world's youngest billionaire is now 26-year-old Dustin Moskovitz, who is 8 days younger than his former Harvard roommate and Facebook cofounder Mark Zuckerberg."