Billionaire = a person possessing assets worth at least a billion dollars (or pounds, etc.).
trillionaire = Somebody whose wealth is greater than one trillion (1012) dollars, or other currency
zillionaire = a person who is extremely rich. A person with seemingly endless amounts of money, a penthouse in Manhattan, a private jet and a ten million dollar house in California is an example of a zillionaire.
jillion. (ˈdʒɪljən) n. (Mathematics) informal an extremely large number or amount: jillions of pounds.