WordNet sense 1 (the fifth and highest element after air and earth and fire and water; was believed to be the substance composing all heavenly bodies):

quintessence

WordNet sense 2 (a medium that was once supposed to fill all space and to support the propagation of electromagnetic waves):

aether

WordNet sense 3 (a colorless volatile highly inflammable liquid formerly used as an inhalation anesthetic):

ethoxyethane, diethyl ether, divinyl ether, ethyl ether, vinyl ether