The Terragen Sphere, sometimes called the Terragen Bubble, is the region of the galaxy that has been colonized or explored by Terragen sophonts, as opposed to regions known only by observation at a distance, or reported to Terragens by xenosophonts such as the Muuh or the Soft Ones. Popular imagination makes this a sphere about 14,000 light years across, with its centre at Old Earth, but this is only very approximately correct, for a variety of reasons. Terragen expansion has been biased towards the more numerous stars in the galactic disk, making for an overall shape that is more oblate than spherical, and of course there are very large volumes within this area that remain unexplored, so the sphere is far from uniform. Also, in some cases Terragens have encountered the remains of wormhole networks created by xenosophonts; the Tunh and Carina wormholes have expanded the Terragen reach by some thousands of light years in those directions.
In fact the true extent of Terragen reach is only a theoretical maximum, and much within it remains untouched or unknown from the point of view of any one Terragen civilization or polity. Even friendly civilizations aligned with the Sephirotics do not necessarily report back regarding each and every explored or inhabited star system, and of course Hider, Ahuman, and Antihuman civilizations may refuse to of share data with the rest of the Civilized Galaxy or with each other.
Finally, just because Terragens have visited a place, even to the extent of colonization, does not mean that they have fully explored it, much less come to understand it. Nevertheless, 'Terragen Sphere' is a widely used expression. It conveys the equivalent of the phrase 'The Known World' as used on Old Earth by various Agricultural Age civilizations, or as used in times since by various Prim and Low-tech civilizations that have limited knowledge of their own planet, moon, or megahab. In recognition of this, some prefer to say 'The Known Galaxy' or 'Known Space' instead.
Other terms for the Terragen Sphere include the Orion Arm Civilisation, or sometimes simply 'Orion's Arm'; however the Terragen Sphere does not occupy the full extent of the Orion Arm or spur, nor is it entirely confined to that arm, so these names are not particularly accurate.