An oligarch is one of the rulers in an oligarchy, which is a small group of people who are in power.
The word oligarch has Greek roots, and comes from oligoi for "few” and arkhein "to rule." When a country is ruled by an oligarchy, power isn't in the hands of one person (like a monarchy) or the people (like a democracy) but a small group of people. An oligarch can be a member of the nobility or a wealthy person. People often say a democracy has oligarchs when rich people appear to have too much influence on a country.