A vast and supposedly impassable energy field surrounding the centre of the galaxy, which the hijacked Enterprise-A crosses in 2287 to reach the planet Sybok believes to be Sha Ka Ree.
The Great Barrier is an immense field of energy that encloses the centre of the galaxy. By the late 23rd century no ship or probe sent into it was known to have returned, and it was widely regarded as a limit that could not be crossed.
In Star Trek V: The Final Frontier, the renegade Vulcan Sybok seizes the USS Enterprise-A and orders it through the barrier, convinced that the mythical paradise of Sha Ka Ree lies beyond. The ship survives the passage and finds a single barren planet.
The world proves to be a prison rather than Eden. The entity confined there poses as God and demands a starship to carry it out through the barrier, until Kirk questions it and a Klingon Bird-of-Prey destroys its manifestation.