Ether is a colorless, highly flammable liquid that was once used as an anesthetic to make patients unconscious during surgery. Before modern anesthetics were developed, doctors would have patients breathe ether vapor through a cloth or mask, and within minutes they would fall into a deep sleep, feeling no pain during operations.

In ancient times, people also used the word for the pure, bright air that the gods breathed, or the substance that filled the heavens beyond Earth's atmosphere. This ancient meaning led scientists to use ether for something they once believed filled all of space: a mysterious invisible substance through which light waves supposedly traveled. Scientists in the 1800s thought this “luminiferous ether” must exist because they couldn't imagine how light could move through empty space. But in 1887, a famous experiment showed that this cosmic ether didn't exist at all.

In chemistry, ether also refers to a whole family of compounds that share a similar molecular structure. These chemical ethers have many uses, from making plastics to serving as solvents that dissolve other substances.

When you see “ether” in older books or historical accounts, it's usually referring to either the anesthetic or that imaginary space-filling substance that turned out not to exist.