magnesia

This word has more than one story. Most often, it means a white mineral substance connected with magnesium, especially magnesium oxide. In older writing, it may refer to a material used in powders, antacids, or laxatives. You may also see it in the fixed phrase "milk of magnesia," although that product is actually magnesium hydroxide, not magnesium oxide.

Less often, the word points to Magnesia, an ancient place-name linked with minerals and with the history of the word "magnet." That older geographic sense now appears mainly in historical writing.

Magnesia is a word with both scientific and historical meanings. In modern English, it usually refers to magnesium oxide (MgO), a white substance used in chemistry, industry, and older medical writing. In past centuries, the word was sometimes used more loosely for related magnesium substances as well, which is one reason it appears in names such as "milk of magnesia." Strictly speaking, however, milk of magnesia is magnesium hydroxide.

The word also carries an older geographic sense. Magnesia was the name of places in the ancient Greek world, especially in Asia Minor and Thessaly. Those place-names became linked with unusual minerals, and from that connection English eventually inherited several related words, including "magnet" and "magnesium." Because of this, "magnesia" can sometimes appear in historical or etymological writing as more than just the name of a chemical substance.

So the word moves between the lab and the ancient world. In one setting, it names a white magnesium compound; in another, it points back to old regions whose names shaped the language of science. Most of the time today, though, if you see "magnesia" on its own, it is referring to the substance rather than the place.