A fence
A fence was a structure designed to mark off certain areas and/or inhibit access to them.
In an alternate 2255, a wired fence surrounded the Riverside Shipyard where the Enterprise was under construction. There was a sign on the fence that said "Authorized Personnel Only." (Star Trek)
Several "forbidden areas" on the planet Rubicun III were surrounded by fences. In 2364, Wesley Crusher jumped over one of these fences while trying to catch a ball. (TNG: "Justice")