Geodesic fold

From Memory Alpha, the free Star Trek reference

A geodesic fold is a temporary gateway through space. They can be created by firing a geodesic pulse, a verteron beam, into the magnetic field of a red giant star. As the geodesic fold opens, a corresponding fold will open in the magnetic field of another red giant, linking the two points. Communication transmissions cannot pass through a geodesic fold.

Geodesic folds, however, emit high levels of geodesic radiation, which standard Federation shields cannot withstand. Therefore, any ship attempting to pass through will have all organic tissue aboard liquefied.

In 2377, DaiMon Nunk hatched a scheme to bring USS Voyager to the Alpha Quadrant, so that he could sell Seven of Nine's nanoprobes. He intercepted the hologram of Reginald Barclay and programmed it to tell the Voyager crew that Starfleet had made advances in shields and radiogenic vaccines that would allow them to pass safely through a geodesic fold.

From his marauder, Nunk created a geodesic fold at a red giant, and another fold opened at red giant 23139 in the Delta Quadrant. However, Barclay, posing as his hologram, was able to bluff Nunk into collapsing the fold before Voyager could pass through. (VOY: "Inside Man")