The Sagittarius Arm is a collection of stars, planets, nebulae, and other spatial objects that form one of many curved "arms" extending from the center of the Milky Way Galaxy.
The Sagittarius Arm seems to extend from the galactic core, a short bit through the Gamma Quadrant, and extend around and towards the center of the Alpha Quadrant. The smaller Orion Arm (or Local Spur), which contains Earth and the Sol system, extends from this arm.
The Sagittarius Arm was shown on a computer panel in astrometrics on the USS Voyager during the later years of its seven-year journey in the Delta Quadrant. (Background Graphic, Star Trek: Voyager)