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 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. (DS9: "Emissary")
Trill scientist Lenara Kahn attempted to create a similar artificial wormhole based on studies of the Bajoran wormhole. Initially, she was only partly successful, as the new wormhole collapsed after a few fractions of a second, but continued work and refinement of her theories eventually led to the creation of the artificial wormhole network, which lessened the Bajoran wormhole's strategic importance. However, as of 2386, it continued to be an active thoroughfare for traffic between the Alpha and Gamma Quadrants. (Star Trek: Arcadia)
In an early continuity of Star Trek: Arcadia, the Bajoran wormhole was "cross-linked" in the mirror universe via a subspace "bridge" to a wormhole in the Delta Quadrant, allowing an attempted invasion of the Alpha Quadrant by the Imperium. It was discovered that the "Prophets" of the mirror universe were more like the primary universe's Pah-wraiths, and responsible for much of the evil there.