A warp-gate, also known as a dimensional gate, was an opening between dimensions.
After Krugar III was destroyed by a comet, a small section of land containing a museum was carried off into space and through what the Curator called "a hole in the stars" into a netherspace he called "Limbo". Over the years, many starships became trapped in Limbo, their pilots unable to escape despite many attempts. After the USS Enterprise and a Klingon battle cruiser became trapped, Spock discovered that the gate could be opened if sufficient force were to be applied at a single point. The Enterprise and the Klingon ship worked together and eventually opened the warp-gate leading from Limbo to their home universe. (TOS comic: "Museum at the End of Time")