Outer Arm is an immense, but little travelled, region on the rim of the galaxy. It is separated from the Perseus arm by the Formidine Rift and Angustia regions, and separated from the rest of the Norma-Outer Arm spiral structure by the Outer Arm Rift.
Prior to the development of hyperspace technology, astronomers could not verify that the Norma Arm observed east of the galactic core, and the Outer Arm observed west of the galactic core where in fact part of the same spiral structure. The discovery of the Boreas region by early deep-space explorers changed this, resulting in the entire spiral structure being renamed to the Norma-Outer Arm.
But after the discovery of the Outer Arm Rift, it has again become customary to refer to the portions of the Norma-Outer Arm that is severed by the rift as the Outer Arm region.