The Bajoran wormhole was a wormhole spanning some 70,000 light years, from the Denorios belt in the Bajor-B'Hava'el system of the Alpha Quadrant, to near the Idran system in the Gamma Quadrant. From 2369 to 2375, it was the only stable wormhole in the galaxy known to the Federation.
The Prophets of Bajoran religion reside in the Celestial Temple, which was discovered to be the wormhole following first contact between the Federation and the wormhole aliens. It was through the wormhole that the Prophets sent their Orbs to Bajor greatly influencing the planet's religion over a 10,000 year period.
In late 2373, to prevent the Dominion from further reinforcing its positions in the Alpha Quadrant Captain Sisko mined the Alpha Quadrant side of the wormhole with cloaked self-replicating mines. In response the Dominion attacked Deep Space 9, causing the start of the Dominion War.
After months of failed attempts the Dominion began to dismantle the minefield and the Federation launched Operation Return to prevent this. The Dominion were able to destroy the minefield, however their vast fleet that entered the minefield disappeared due to the intervention of the Prophets, on the prompting of Captain Sisko.
In an alternate universe where the Dominion won the Dominion War, the USS Defiant was during in the wormhole during Operation Return attempting to prevent the Dominion reinforcements from reaching the Alpha Quadrant.
Following the end of the battle between the Prophet and the Kosst Amojan the wormhole opened, creating gravimetric distortions that pushed the station towards it. At the same time a wave of orbs exited the wormhole seeking out those "of Bajor" to transport them back into the wormhole. When the orbs disappeared so did the wormhole, severing the connection between the Alpha and Gamma Quadrants.
Despite this the Dominion planned to build a new station at the former site of the wormhole. They wished to keep the wormhole's disappearance a secret to prevent any rise in resistance against the Dominion.