The Collaborative International Dictionary of English v.0.48:

Millionaire \Mil`lion*aire"\ (?; 277), n. [F. millionnaire.]
   One whose wealth is counted by millions of francs, dollars,
   or pounds; a very rich person; a person worth a million or
   more. [Written also millionnaire.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

millionaire
    n 1: a person whose material wealth is valued at more than a
         million dollars

Moby Thesaurus II by Grady Ward, 1.0:

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