Meridian was a planet in the Gamma Quadrant's Trialus system that existed between dimensions and periodically shifted in and out of those dimensions. Millenia ago, the planet became the new home of memebers of an expidition that stranded there.
The Trialus sun's core periodically built to a cascade effect at the quantum level, causing Meridian to shift from one parallel universe to another. The planet appeared in regular space every sixty years. When in the other dimension the planet's thirty inhabitants existed as pure consciousness and the planet and everything on it disappear from normal space. This had been the case for thousands of years prior to the USS Defiant arrival in 2371.
By 2371, however, the planet had been undergoing changes for several centuries and its time in normal space had been growing shorter with each visit. At that time Meridian would manifest in normal space for only twelve days, and it was predicted that within a few centuries their stay would last only minutes.
That year, the USS Defiant, on a mission of exploration in the Gamma Quadrant, was present when Meridian returned to normal space for the first time since 2311. The crew of the starship studied the Trialus sun along with the scientists of Meridian and discovered an instability that was causing the shortened stay of the planet in normal space. With the help of the Defiant the Meridians were able to stabilize the star and expected their next manifestation to last thirty years, long enough to begin having families again. Jadzia Dax attempted to remain on Meridian as it shifted realities, but was unable to as her presence would have destroyed the planet. (DS9: "Meridian")