The term is most often used in Britain, in which context it includes leading politicians, senior civil servants, senior barristers and judges, aristocrats, Oxbridge academics, senior clergy in the established Church of England, the most important financiers and industrialists, governors of the BBC, and the members of and top aides to the royal family. For example, candidates for political office are often said to have to impress the “party establishment” in order to win endorsement. The term in this sense is sometimes mistakenly believed to have been coined by the British journalist Henry Fairlie, who in September 1955 in the London magazine The Spectator defined that network of prominent, well-connected people as “the Establishment”, explaining:

“By the Establishment, I do not only mean the centres of official power—though they are certainly part of it—but rather the whole matrix of official and social relations within which power is exercised. The exercise of power in Britain (more specifically, in England) cannot be understood unless it is recognized that it is exercised socially.”
Following that, the term, the Establishment, was quickly picked up in newspapers and magazines all over London, making Fairlie famous. However, the term The Establishment, had been used by Ralph Waldo Emerson in a similar fashion, a century earlier. Nevertheless, the Oxford English Dictionary would cite Fairlie’s column as its locus classicus.

However, author and professor Carroll Quigley of Georgetown University, in his book The Anglo-American Establishment, used the term much more specifically than did Fairlie. In that book (copyright date 1981), according to an out-of-print edition:
Quigley exposes the secret society’s (sic) established in London in 1891, by Cecil Rhodes. Quigley explains how these men worked in union to begin their society to control the world. He explains how all the wars from that time were deliberately created to control the economies of all the nations.

That society was established by Cecil Rhodes in 1891 and, following Rhodes’ death in 1902, was carried on by Alfred Milner, which society, Quigley refers to as the Milner Group, but sometimes referred to as the Round Table movement. That group, with significant American input, would, following the Paris Peace Conference of 1919, establish and control the Royal Institute for International Affairs, later to become known as Chatham House.
Much more generally, this use of the word, Establishment, may have been influenced by the British term, established church, for the official church of Great Britain. The term was then found useful in discussing the power elites in many other countries. The English word is now used as a loanword in many other languages.