ether — akasha
The Sanskrit word akasa, translated as both "ether" and "space," refers specifically to the vibratory element that is the subtlest in the material world. (See elements) It derives from a, "towards" and kasha, "to be visible, to appear."
Akasha is the subtle "background" against which everything in the material universe becomes perceptible. "Space gives dimension to objects; ether separates the images," Paramahansa Yogananda said. "Ether-permeated space is the boundary line between heaven, or the astral world, and earth," he explained. "All the finer forces God has created are composed of light, or thought-forms, and are merely hidden behind a particular vibration that manifests as ether."