What is the Meaning & Definition of oligarchy

The oligarchy is a word in our language refers to a type of form of Government that is characterized because the power is exercised by a very small group of individuals who normally share the same rank social and economic.
It is worth mentioning is that the concept is also used to refer to the set of entrepreneurs or people who have wealth, which usually grouped and joint action to defend their interests and property.
The origin of this form of Government goes back to the ancient Greece, in which would be born as a deformation of Government otherwise: the aristocracy.
It is worth mentioning is that in those days the aristocracy grouped individuals more able to govern, while, when this system began to deviate, being perpetuated by the blood descent, who headed the Government already were not the best and most capable but the owners of the resources. Thus arose the concept of oligarchy.
The oligarchy does not accept in any way the mobility of social class.
The Athenian philosopher Plato was one of the many classic authors that spoke of the oligarchy for example citing the Government of the thirty tyrants in their city. Called it that way because it just was composed of 30 judges who assumed absolute power after the Peloponnesian War, going on to the democratic form of Government, and imposed an oligarchic.
Meanwhile, will be named as oligarch that individual who integrates the oligarchy.
Historically, the oligarch painted it is as a millionaire individual, which ignores the ethical or moral, because if it is necessary it is able to make inroads into corrupt and violent acts to preserve their status, and that uses its resources to press politically to whom it may concern and thus to enforce interests, on the one hand, and on the other hand to further extend its riches.
The political ideology of the left has traditionally been the main opponent of the oligarchy around the world.
Opposite of this form of Government is democracy, that as we know is the way Government attributed the supreme power to the people, being he who gives you power through mechanisms for direct participation and legitimacy to their representatives.