Intermittent cyclical vortex

From Memory Alpha, the free Star Trek reference.

An intermittent cyclical vortex is a type of interspatial flexure, or more generally a wormhole. It is characterized by opening at varying locations and times.

One such vortex was frequently used by telepaths to escape Devore territory. The USS Voyager searched for the wormhole in 2375, so as to bear a number of Brenari refugees and its own telepathic crew members to safety. They obtained data on the wormhole from the scientist Torat, which seemed to show that the wormhole appeared at random.

Captain Kathryn Janeway realized that each of the wormhole's parameters had a counterpoint in subspace, and so was able to predict the wormhole's next appearance in the Tehara system. However, the Devore Kashyk, who had apparently defected to Voyager earlier, revealed his true agenda as one of destroying the wormhole, to prevent any more refugees from escaping. Janeway though had anticipated his betrayal, and misdirected him away from the wormhole's true location. Thus, the Brenari escaped and the wormhole vanished before the Devore could reach it. (VOY: "Counterpoint")