Source : Webster's Revised Unabridged Dictionary (1913)

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}.]

Source : WordNet®

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