The Delta Quadrant is the farthest quadrant of the Milky Way from the Alpha Quadrant. It would be located between 12 and 3 o'clock positions if the plane of the galaxy were seen as a clock face with the 6 o'clock position bisecting the Sol system.
Due to the immense distance between the Alpha and Delta Quadrants (an Intrepid class starship from Earth would take 30 years to reach the edges of the quadrant at its maximum warp velocity), very little was known about this region of space until 2371, when the starship USS Voyager was pulled into the quadrant by an alien force called the Caretaker. When contact was reestablished with the starship in 2374, hundreds of kiloquads of data on the region was received, increasing the Federation's knowledge of the quadrant immensely.
The major power in the Delta Quadrant is the Borg Collective. Borg space covers over two-thirds of the quadrant in irregular arrangements, from the border with the Beta Quadrant out to the galactic rim. Other alien species that control significant areas of the quadrant include, among others, the Vidiians, the Devore, the Kazon, the Voth, the Hirogen, the Hierarchy, and the Malon. Species 8472 has also established a presence in the Delta Quadrant.