The Umbra is the collective term for several related magickal parallel dimensions. The Umbra goes by many names, depending on who you ask to describe it.
To the werecreatures, it is simplly the Umbra, the shadow of the moon in eclipse, a place they can reach by "stepping sideways", and which is itself divided into several sorts of spirit realms. Here you will find the Afterlife of each major religion, the Courts of the major pantheons of pagan deities, a realm of eternal battle where restless spirits of soldiers can fight and refight the wars they gave their lives to win in vain, a vast Wilderness known as the Spirit Wilds wich is untouched by the ravages of humans and their foul technology, and empty plains of spiritual desolation.
To the Mages, it is the Tellurian, a series of nine dimensions mystically tied to the nine planets of Earth's original solar system, and to the nine spheres of Magick. Here one will find the Avatar Storm, a sort of tornado of lost souls, or of the shattered shards of the very soul of the universe, pieces of which periodically spin out into our own dimension to attach themselves to the souls of normal humans and allow them to become awakened as mages.
To the Vampires, it is the Shadowlands or Wraithlands, a realm of eternal darkness where the most mystically adept of vampires can hide from their mortal foes and the scorching light of day. Here one will find the pale shadow of the City of Enoch, the first city, whose real-world equivalent was destroyed by the Flood, but whose spiritual twin survives in limbo.
To the human scientists and most alien races, it is Hyperspace, a parallel dimension where points of space are somehow much closer together, a swirling vortex of frozen orange lightning and unpredictable gravitational anomalies, a shortcut between the stars for those brave enough to plumb its depths, a highway across the galaxy for those seeking to build empires of trade or raw political power.
Long before Gehenna, or even the flight of Sputnik, Mages of the Void Engineers and the Sons of Ether probed the limits of the Umbra in great ships known as Umbral Dreadnoughts. It was they who gave the secret of spaceflight to the rest of humanity, packaged as hyperspace travel so their unawakened minds could grasp even the simplest aspects of how it worked. But it was they who discovered the Horadrim homeworld and stole their last surviving children, causing the Horadrim to unleash the Bugs on humanity in retribution.
All of these versions of the Umbra are separated from normal reality by the Gauntlet, a metaphysical membrane that shields reality from unreality, and vice versa. It is thicker in some places, such as scientific laboratories and shopping malls, and thinner in others, such as ancient temples and sacred forest groves. The enormous paradox backlash caused by Bruce Von Eisenstein fighting a Bug Queen in Rio De Janiero in 2001 enormously damaged the Gauntlet, and the leakage of unreality into reality as a result touched off Gehenna. The larger Bug Asteroid of 2198 not only devastated Earth itself, but it tore right through the weakened Gauntlet and wrecked the portions of the Umbra parallel to Earth itself, rendering the Shadowlands and Spirit Wilds and the City of Enoch uninhabitable for Vampire and Werewolf alike.
The Umbra parallel to most other planets is far more bleak and featureless than the densely symbolic parts near Earth itself. They have not had milennia of psychic baggage accumulate to fill them with memories, souls, lost socks and forgotten dreams. So they are harder to reach by stepping sideways, and they are rarely visited these days by even the most spiritual of werecreatures, or the most desperate of vampires.