plutocracy

Concise Oxford English Dictionary © 2008 Oxford University Press:
plutocracy /pluːˈtɒkrəsi/

▶noun (pl. plutocracies) government by the wealthy.

■ a state or society governed in this way.

■ an elite or ruling class whose power derives from their wealth.

– derivatives
plutocratic adjective,
plutocratically adverb.

– origin C17: from Gk ploutokratia, from ploutos ‘wealth’ + kratos ‘strength, authority’.