Oligarchy is a form of power structure in which power effectively rests with a small number of people. These people could be distinguished by royalty, wealth, family ties, education, corporate, religious or military control. Such states are often controlled by a few prominent families who typically pass their influence from one generation to the next, but inheritance is not a necessary condition for the application of this term.
Throughout history, oligarchies have often been tyrannical (relying on public obedience and/or oppression to exist). Aristotle pioneered the use of the term as a synonym for rule by the rich, for which another term commonly used today is plutocracy.