The Sagittarius Arm was a collection of stars, planets, nebulae, and other spatial objects that formed one of many curved "arms" extending from the center of the Milky Way Galaxy.
The Sagittarius Arm seemed 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 contained Earth and the Sol system, extended 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. (VOY: "Endgame" display graphic )