What is the X in X-mas?
late 14c., "upper regions of space," from Old French ether and directly from Latin aether "the upper pure, bright air," from Greek aither "upper air; bright, purer air; the sky," from aithein "to burn, shine," from PIE root *aidh- "to burn" (see edifice).
In ancient cosmology, the element that filled all space beyond the sphere of the moon, constituting the substance of the stars and planets. Conceived of as a purer form of fire or air, or as a fifth element. From 17c.-19c., it was the scientific word for an assumed "frame of reference" for forces in the universe, perhaps without material properties. The concept was shaken by the Michelson-Morley experiment (1887) and discarded after the Theory of Relativity won acceptance, but before it went it gave rise to the colloquial use of ether for "the radio" (1899).
The name also was bestowed c.1730 (Frobenius; in English by 1757) on a volatile chemical compound known since 14c. for its lightness and lack of color (its anesthetic properties weren't fully established until 1842).
ether e·ther (ē'thər)
Any of a class of organic compounds in which two hydrocarbon groups are linked by an oxygen atom.
An anesthetic ether, especially diethyl ether.
ethyl oxide n.
See diethyl ether.