The Outer Arm Rift is a marked discontinuity in the Norma-Outer Arm spiral structure. It is an expanse of low density systems, but with modern FSD technology it poses little danger to travelers heading across it.
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.