The Periphery is the ever-expanding wavefront of Terragen exploration and colonisation. It is in fact the furthest layer of the Outer Volumes, beyond the furthest wormholes. Because the speed of expansion of the Wormhole Nexus is limited to 0.7c, there is a large ring-shaped section of the Explored Galaxy that is not yet connected. Only the fastest exploration and colony ships have reached this far.
The location of the Periphery changes over time, as the exploration wavefront moves ever farther out, and the Wormhole Nexus expands over time. But the exploration wavefront is moving faster than the expansion of the Nexus, so the Periphery is getting gradually larger over time. In some locations small, independent wormhole nexii have been created, not yet connected to the main nexus. Some of the oldest of these independent networks, like the Lirantiq nexus, have been connected to the main network and are now in the mainstream civilisation.
Periphery worlds can only be reached by taking a stargate to the furthest system out, then obtaining a relativistic ship (by building one, buying one or receiving one as a gift; rarely by stealing) and making for the unknown. Of course, there will always be someone who has gone further, but the periphery is so vast that there is enough for everyone. Some ships are headed for the galactic core, or the globular clusters, or even aim to leave the galaxy entirely. Many of these have abandoned their quest and established colonies at the edge of the Terragen Sphere.
There have been several major wars on the Periphery in the last millennium or so, but the only one that is generally known as the Periphery War is the one between the Arion Ascendancy and the Periphery Pact.
Carina-Vela Outer Volumes - Text by M. Alan Kazlev Large region of space in the outer volumes, not affiliated with any of the main empires. The Andian Mission today remains a highly regarded support organization here.
Clive's Brain - Text by Steve Bowers Cybyotan world in the Vela-Puppis border periphery. The alien Cybyotan phylum of interconnected mobile and sessile autotrophs and heterotrophs has on this planet developed a single shared datasphere which is home to a single entity, known as Clive.
Eta Carinae - Text by Chris Clowes A binary star system comprising of a peculiar, erratically fluctuating Luminous Blue Variable and an O-type supergiant star. One of the most luminous stellar objects of our Galaxy.
Historical Diaconcentrism - Text by M. Alan Kazlev The tendency for the peripheral regions to be historically and technologically backward - "living fossils" - in relation to the inner sphere (owing to the amount of transit time - usually many centuries - required to get there). Moving from the core to the periphery is in a sense like moving back in time. This phenomenon breaks down as the outer volume worlds develop their own cultures and civilisations.
Laughter Nebula - Text by Anders Sandberg One of the five nebulae in the volume close to the War and Peace Nebula named after the Rational Virtues (Laughter, Curiosity, Zeal, Reflection and Compassion) in Konensis Mysticism.
Lernaea - Text by Thorbørn Steen A Caretaker God who guards a number of life-bearing planets on the current frontier of the Terragen Sphere. The life on these planets may or may not be of Terragen origin.
Pluton Volume, The (and the Epp War) - Text by Steve Bowers and Rakuen Wealthy region in the Perseus Arm, with a large xenosophont population; site of the conflict between the Epp and their Methanoid allies, and the Perseus Princes (with the aid of the System of Response).
Sagittarius Periphery Wars - Text by M. Alan Kazlev A serious of skirmishes, blights, and territorial wars along the Sagittarius frontier during the middle Empires period. The potentially worst of these were defused by the Specialist in a series of oft-recounted real-life interventions.
Utochishte System - Text by Domagoj K Colony on the periphery of Terragen Space which was isolated for more than a thousand years and developed a complex and idiosyncratic culture. This resulted in the creation of an extensive virtual world, divided into millions of mini-universes.