WordNet (r) 3.0 (2006):

billionaire
    n 1: a very rich person whose material wealth is valued at more
         than a billion dollars

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "billionaire":
   Daddy Warbucks, bloated plutocrat, capitalist, fat cat,
   man of means, man of substance, man of wealth, millionaire,
   moneybags, moneyed man, multimillionaire, nabob, parvenu,
   plutocrat, rich man, richling, warm man, wealthy man