What is an "oligarchy"?

http://Dictionary.com  defines it as "a form of government in which all power is vested in a few persons or in a dominant class or clique"

The online Encyclopedia Britannica defines "oligarchy" as "despotic power exercised by a small and privileged group for corrupt or selfish purposes."

The Cambrige Dictionary defines "oligarchy" as "the fact of a country being ruled or controlled by a small group of powerful people."

The Merriam-Webster Dictionary defines "oligarchy" as "a government in which a small group exercises control especially for corrupt and selfish purposes."