ethereal

In ancient Greek cosmology, aithēr was the fifth element: the pure bright substance that filled the heavens above the cloudy air. Aristotle argued in "On the Heavens" (350 BC) that the planets and stars were made of aithēr because they moved in perfect circles without change or decay. Below it was air, fire, water, and earth; above it was aithēr, unburning and unchanging.

The word's root was aithein, to blaze or kindle. So aithēr was the blazing stuff, the upper fire that was not like ordinary fire. The Stoics and Roman writers inherited the concept through Cicero, Virgil, and Ovid, who wrote aether as the high dwelling of the gods. For Virgil, souls of the righteous rose to the aether after death.

Medieval Latin kept aetherius as a learned adjective: celestial, divine, located in the upper regions of the universe. English poets borrowed it in the 1590s as ethereal, applying it first to things literally of the upper air, then quickly to things that seemed too delicate for this world. Milton used ethereal twenty times in "Paradise Lost" (1667), almost always to describe light or angelic motion.

By the 19th century, ethereal had settled into its modern sense: light, delicate, unworldly, almost transparent. The Romantic poets found in it a perfect word for beauty that seemed too fine to last. The cosmological machinery behind it fell away, and what remained was the quality of the upper air itself: clear, bright, beyond ordinary weight.