The Gamma Quadrant is one of the four quadrants of the Milky Way Galaxy. The inhabitants include (but are not limited to) the Changelings, Vorta, Jem'Hadar and T-Rogoran, all of whom are members of the Dominion, an interstellar empire that controls most of the quadrant.
In 2369, Benjamin Sisko and Jadzia Dax discovered a wormhole near Bajor and space station Deep Space 9 that connected the Alpha Quadrant to the Gamma Quadrant, the discovery of which eventually led to the Dominion War.
After the Dominion War, the Dominion permitted Federation starships to renew the exploration of the Quadrant. Commander Elias Vaughn commanded the USS Defiant in the first exploratory mission in 2376. The crew of the Defiant made first contact with the Vahni Vahltupali, the Yrythny, the Cheka, the D'Naali, the Nyazen and the USS Valkyrie, which had been assimilated by the Borg.