By definition, an oligarchy is a "government by the few" where there is a chance that despotic power is displayed and utilized by a small, privileged bunch of people - usually for corrupt and selfish purpose. This word comes from a term used by the Greek philosopher Aristotle where he used the term oligarchia to designate a rule by a few bad people who are set apart from the rest of society by factors like religion, kinship, financial status, prestige, or language. These elites will act in the interest of their own class at the expense of other groups in society. As mentioned in Encyclopedia Britannica, the term oligarchy is very important in the contemporary United States of America since some political scientists argue that the modern version of the USA is an oligarchy since great wealth inequality allows corporations and particular financial elites to influence public policy to their advantage against the preferences of the majority.
There is a study by Princeton University Prof Martin Gilens and Northwestern University Prof Benjamin I Page that says the United States is a civil oligarchy. The work Oligarchy by Jeffrey A. Winters also describes the United States today as such an entity & describes what a civil oligarchy is: a system where the guaranteed security of property does not stop wealthy people from gaming the political process to protect their sources of wealth and to give themselves massive economic advantages. They can also work on imposing economic burdens and responsibilities onto the poor and middle class while lowering their own responsibilities for the nation's problems.
What is the main issue with all of this? Well, like I have discussed before, many modern countries seem to be returning to a modern form of feudalism some political scientists and experts refer to as neofeudalism. In a previous article, I discussed how working gigs where you depend on corporations for clients and do some unpaid work and pay a fee for the 'privilege' of doing a job is similar to the concept of peasant work during feudalism. On top of that, many corporations are buying up land and renting it out to people to create a work of corporate landlords where only wealthy corporate elites and their children are able to own private property. Now, having countries that are supposed to be democratic become indirect oligarchies helps in the process of bringing back modern feudalism. You create a world where the wealthy have power to influence our laws while giving people enough of an illusion that they still have a say in governance to keep them docile is a decent way to subtly bring back the feudal era. The rich get to have more and more of a say in government while reducing economic opportunities to a dying middle class and claim that people just 'work hard enough', they could wealthy too - something that becomes nearly impossible as issues like the rise of civil oligarchies pop up! Plus, plenty of oligarchs now have more access to data that allows them to better shape public discourse and propaganda to shape their own interests. Plus, the oligarchy grows as more of them purchase media companies that allows them to have control over the news & information that common people had - similar to how information in the feudal age was shaped by the ruling families. Oligarchy seems to be one of the steps towards a truly feudal future for us...