Isolationism was the tendency of a state to withdraw behind its own borders and to minimize or cut contact with other races.
After they lost the Earth-Romulan War and the establishment of the Romulan Neutral Zone in 2161, the Romulans went into isolation for over a hundred years. (TOS: "Balance of Terror") They did so a second time for over fifty years after the Tomed Incident in 2311. (TNG: "The Neutral Zone")
Likewise, the 24 Great Houses of the Klingon Empire isolated themselves for almost a century prior to the outbreak of the Federation-Klingon War in 2256, with only few encounters between the Federation and the Klingons. (DIS: "The Vulcan Hello", "Battle at the Binary Stars")