One of the galaxy's four quadrants, containing the Klingon Empire and the Romulan Star Empire as well as part of the Federation.
The Beta Quadrant is one of the four quadrants into which Starfleet divides the Milky Way galaxy. It lies next to the Alpha Quadrant, and Earth sits close to the border between the two. Part of the United Federation of Planets lies within it, together with the territory of the Klingon Empire and the Romulan Star Empire, two of the Federation's most important rivals and allies.
Many key events in Star Trek happen in the Beta Quadrant, including the long history of tension along the Romulan Neutral Zone and the Federation's shifting relationship with the Klingons, from open war to alliance. The Klingon homeworld of Qo'noS and the Romulan homeworld of Romulus both lie in this quadrant.
The Beta Quadrant is often grouped with the Alpha Quadrant when characters speak of home, especially in Deep Space Nine and Voyager. Its great powers played a central role in the Dominion War, when the Klingons and Romulans joined the Federation against the Dominion.