In the Star Trek universe, the Alpha Quadrant is 1/4 of the Milky Way. The largest power in the quadrant is United Federation of Planets. The Cardassian Union also resides here, so prior to the Dominion War, they were a sizable power as well. Additionally, during the war the Dominion had a major foothold there, thanks largely to the Cardassians becoming a member. In 2375, Federation-Klingon-Romulan alliance were joined by the Cardassian fleet, resulting in the defeat of the Dominion, bringing peace back to the quadrant.
Other powers in the quadrant include the Ferengi, the Bajorans, and relatively small portions of the Klingon Empire and the Romulan Star Empire. Planets include Bajor, Betazed, Cardassia, Earth, Ferenginar, and Trill.
The Alpha Quadrant is home to the only known stable wormhole, the Bajoran Wormhole, which leads roughly 70 000 light-years to the Idran system in the Gamma Quadrant. It was through this which the Alpha Quadrant species met the Dominion, ultimately resulting in the Dominion war.
The Alpha Quadrant is usually shown at the lower-left on maps of the galaxy.