According to the Oxford Dictionary the word plutocracy means: "government by the wealthy"
According to the Etymology Dictionary the word plutocracy comes from the name of the Greek god of wealth, Plouton. "In ancient Greek religion and myth, Pluto was a name for the ruler of the underworld; the god was also known as Hades, a name for the underworld itself. This deity has two major myths: in Greek consmogony, he received the rule of the underworld in a three-way division of sovereignty over the world, with his brothers Zeus ruling Heaven and Poseidon the Sea; and he abducts Persephone to be his wife and the queen of his realm. In other myths, he plays a secondary role, mostly as the possessor of a quest-object." in words of Wikipedia.
Other words referring to pluto are: plutocrat, plutocratic and plutocratically (according to Oxford Dictionary). We can see the use of the word plutocracy in this article