Definition: a group of social, economic, and political leaders who form a ruling class (as of a nation); a controlling group
When did we begin to shake our fists in impotent rage, while decrying “the establishment”? It’s a bit hard to say, since establishment has had a fair number of meanings over the centuries, some of which bleed into each other, but lexicographers have traced the use of “the Establishment” as a social group to at least the early 1920s. In politics we have been referring to “establishment candidates” for over 50 years; an article in the Pittsburgh Post-Gazette from 1964 contains the line “Even the masses, he said, are content to give their votes to “establishment” candidates in exchange for small material concessions.”
Even so, McConnell was met with scattered boos from the convention floor, another sign that this gathering is in the hands of Trump and not the party establishment.
—Dan Balz, The Washington Post, 19 July 2016