Oligarchy (from Greek (oligarkhía); from (olígos) 'few', and ? (arkho) 'to rule or to command') is a form of power structure in which power rests with a small number of people. These people may or may not be distinguished by one or several characteristics, such as nobility, fame, wealth, education, or corporate, religious, political, or military control.
Throughout history, oligarchies have often been tyrannical, relying on public obedience or oppression to exist. Aristotle pioneered the use of the term as meaning rule by the rich, for which another term commonly used today is plutocracy. In the early 20th century Robert Michels developed the theory that democracies, like all large organizations, have a tendency to turn into oligarchies. In his "Iron law of oligarchy" he suggests that the necessary division of labor in large organizations leads to the establishment of a ruling class mostly concerned with protecting their own power.
The exclusive consolidation of power by a dominant religious or ethnic minority has also been described as a form of oligarchy. Examples of this system include South Africa under apartheid, Liberia under Americo-Liberians, the Sultanate of Zanzibar, and Rhodesia, where the installation of oligarchic rule by the descendants of foreign settlers was primarily regarded as a legacy of various forms of colonialism.
The modern United States has also been described as an oligarchy because some literature has shown that economic elites and organized groups representing special interests have substantial independent impacts on U.S. government policy, while average citizens and mass-based interest groups have little or no independent influence.
A business group might be defined as an oligarchy if it satisfies all of the following conditions:
George Bernard Shaw defined in his play Major Barbara, premiered in 1905 and first published in 1907, a new type of Oligarchy namely the intellectual oligarchy that acts against the interests of the common people: "I now want to give the common man weapons against the intellectual man. I love the common people. I want to arm them against the lawyer, the doctor, the priest, the literary man, the professor, the artist, and the politician, who, once in authority, is the most dangerous, disastrous, and tyrannical of all the fools, rascals, and impostors. I want a democratic power strong enough to force the intellectual oligarchy to use its genius for the general good or else perish." 
Since the collapse of the Soviet Union and privatization of the economy in December 1991, privately owned Russia-based multinational corporations, including producers of petroleum, natural gas, and metal have, in the view of many analysts, led to the rise of Russian oligarchs. Most of these are connected directly to the highest-ranked government officials, such as the President.
The Ukrainian oligarchs are a group of business oligarchs that quickly appeared on the economic and political scene of Ukraine after its independence in 1991. Overall there are 35 oligarchic groups. 
Some contemporary authors have characterized current conditions in the United States as oligarchic in nature. Simon Johnson wrote that "the reemergence of an American financial oligarchy is quite recent", a structure which he delineated as being the "most advanced" in the world. Jeffrey A. Winters wrote that "oligarchy and democracy operate within a single system, and American politics is a daily display of their interplay." The top 1% of the U.S. population by wealth in 2007 had a larger share of total income than at any time since 1928. In 2011, according to PolitiFact and others, the top 400 wealthiest Americans "have more wealth than half of all Americans combined."
In 1998, Bob Herbert of The New York Times referred to modern American plutocrats as "The Donor Class" (list of top donors) and defined the class, for the first time, as "a tiny group--just one-quarter of 1 percent of the population--and it is not representative of the rest of the nation. But its money buys plenty of access."
French economist Thomas Piketty states in his 2013 book, Capital in the Twenty-First Century, that "the risk of a drift towards oligarchy is real and gives little reason for optimism about where the United States is headed."
A 2014 study by political scientists Martin Gilens of Princeton University and Benjamin Page of Northwestern University stated that "majorities of the American public actually have little influence over the policies our government adopts." The study analyzed nearly 1,800 policies enacted by the US government between 1981 and 2002 and compared them to the expressed preferences of the American public as opposed to wealthy Americans and large special interest groups. It found that wealthy individuals and organizations representing business interests have substantial political influence, while average citizens and mass-based interest groups have little to none. The study did concede that "Americans do enjoy many features central to democratic governance, such as regular elections, freedom of speech and Association, and a widespread (if still contested) franchise." Gilens and Page do not characterize the US as an "oligarchy" per se; however, they do apply the concept of "civil oligarchy" as used by Jeffrey Winters with respect to the US. Winters has posited a comparative theory of "oligarchy" in which the wealthiest citizens - even in a "civil oligarchy" like the United States - dominate policy concerning crucial issues of wealth- and income protection.
Gilens says that average citizens only get what they want if wealthy Americans and business-oriented interest groups also want it; and that when a policy favored by the majority of the American public is implemented, it is usually because the economic elites did not oppose it. Other studies have questioned the Page and Gilens study.
In a 2015 interview, former President Jimmy Carter stated that the United States is now "an oligarchy with unlimited political bribery" due to the Citizens United v. FEC ruling which effectively removed limits on donations to political candidates. Wall Street spent a record $2 billion trying to influence the 2016 United States presidential election.