The Collaborative International Dictionary of English v.0.48:

Plutocracy \Plu*toc"ra*cy\ (pl[-u]*t[o^]k"r[.a]*s[y^]), n. [Gr.
   ploutokrati`a; plou^tos wealth + kratei^n to be strong, to
   rule, fr. krato`s strength: cf. F. plutocratie.]
   A form of government in which the supreme power is lodged in
   the hands of the wealthy classes; government by the rich;
   also, a controlling or influential class of rich men.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plutocracy
    n 1: a political system governed by the wealthy people