Plutocracy
Etymology
From Greek ploûtos ("wealth") plus -kratía ("power, rule"). The word says plainly what it means: wealth as the organizing principle of who governs. It entered English in the seventeenth century, almost always as a criticism rather than a neutral description.
noun · /ploo-TOK-ruh-see/
Rule or controlling influence by the wealthy, whether openly or behind the scenes.
What it names
It names the quiet mechanism by which money converts into political power, so that wealth itself becomes a form of government.
Example
When a single donor’s preferences could sink any bill, critics said the country had drifted into plutocracy.
Related idea
Oligarchy is rule by a small group for whatever reason; plutocracy specifies the reason is wealth. Nearly every plutocracy is also an oligarchy, but not the reverse.