What is The Establishment?

Legal Definition
The Establishment generally denotes a dominant group or elite that holds power or authority in a nation or organization. The Establishment may be a closed social group which selects its own members or specific entrenched elite structures, either in government or in specific institutions.

The American Sociological Association states that the term is often used by those protesting a small group that dominates a larger organization. For example, in 1968 a group of academics set up the "Sociology Liberation Movement" to repudiate the leadership of the American Sociological Association, which they referred to as the "Establishment in American sociology".

In fact, any relatively small class or group of people having control can be referred to as The Establishment; and conversely, in the jargon of sociology, anyone who does not belong to The Establishment may be labelled an "outsider".
-- Wikipedia