A huge expanse explored by craft manned by Sons of Ether, Void Engineers & others. Difficult to navigate. The air is breathable to some mages but not to others.
The first 2 editions put this in the nearer reaches of the Deep Umbra. For Revised Edition (Infinite Tapestry) 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. Full of ether. Another name for Etherspace is the Horizon or True Horizon.