The Delta Wormhole was a semi-stable wormhole discovered by the United Federation of Planets in the late 24th century which allowed virtually instantaneous travel between the Alpha and Delta Quadrants. The Alpha Quadrant terminus was near the Romulan Neutral Zone, while the Delta Quadrant end opened into the F'asa sector not far from the Beta Quadrant border. (Starfleet Simulation Group)
When an energy being called The Entity passed through it in 2373, the wormhole destabilized, rendering the wormhole impassable except for limited periods. In the aftermath, Starfleet constructed a starbase at the Alpha Quadrant mouth of the wormhole and continued in its efforts to reopen the passage. Eventually, brief communication was restored between the quadrants.