The spirit world (actually a series of worlds). Plural Umbrae. Also Otherworlds, Infinite Tapestry. I keep to the 1st Edition idea that when mages Awaken they discover the world is larger than they thought. The Umbra is part of the world that they didn't know of before. It isn't "unreal". Time and space can work differently in the Umbra. Time may pass more quickly or slowly from Earth. Thinking certain thoughts while holding still can move a person a long distance while much travel may lead nowhere.
Pg. 72 1st ed core "Mages often venture into the
Umbra out of curiosity, and travel to distant Realms in search of
knowledge."
Pg. 180 1st ed core "The rules for magick
use change when a mage leaves physical reality and enters the Umbra, as
the enlightened are often wont to do."
Pg. 239 1st ed
core "Mages frequently journey to the Near Umbra."
Pg. 240
1st ed core "Storytellers should make the Near Umbra
whatever they want and need it to be."
Pg. 30 Infinite Tapestry mentions "Umbral tidal motion". Pg. 21 Infinite Tapestry has good ideas on how to pass between different barriers of layers and areas in the Umbra (besides Earth's Gauntlet).
Infinite Tapestry states a traditional Hermetic cosmology puts the Low Umbra inside the Earth, the Middle Umbra along the surface of the Earth and the High Umbra in the skies above. Mercury, Venus & Mars drift through the Horizon along with Horizon Realms. Some say the Earth is the Shard Realm of Prime. It is noted this cosmology is very old yet keeps proving accurate.
Umbral layers & makeup was changed for Revised Edition in Infinite Tapestry. Before that Etherspace was assumed to be the nearer part of the Deep Umbra. Afterwards, the First Horizon roughly corresponds to the limit of Earth's atmosphere. Etherspace starts there. The limit of Etherspace roughly corresponds to the Asteroid Belt and is called Far Horizon. Great winds are said to blow through there. Another name for Etherspace is the Horizon or True Horizon. Far Horizon is hard to get through, leads to Deep Umbra/Deep Universe (Technocracy)/Void. First 2 editions described the Horizon as a wall. Revised says Horizon Realms drift through the spacious Horizon.
Infinite Tapestry says mages must acclimate after returning from the Umbra. Infinite Tapestry pg. 34 gives new traits for Totems: Essence Shield & Disembodiment Shield. Also other benefits for Umbral travel.