One of the four Quadrants of the Milky Way Galaxy. It's inhabitants include the United Federation of Planets, the Cardassian Union, and parts of the Klingon and Romulan Star Empires.
A wormhole was found near planet Bajor and space stattion Deep Space 9 that connected the area with the Gamma Quadrant, the discovery of which eventaully led to the Dominion War.