• Magnesia, deriving from the Macedonian tribe name Magnetes, is the name of the southeastern area of Thessaly in central Greece.
• Magnesia was also the name of two cities in ancient Lydia (modern Turkey) founded by Greek Magnesians.
  • One of them, Magnesia ad Sipylum is famous as the site of the Battle of Magnesia. Its ruins lie close to present day Manisa.
  • The other was Magnesia ad Maeandrum which lay within Ionia, but because it had been settled by Aeolians from Magnesia of Greece, was not accepted into the Ionian League.
• In geology, magnesia is a natural form of magnesium oxide, also called periclase.
• In alchemy and early chemistry, magnesia alba was the name for an indefinite hydrated mixture of magnesium hydroxide and magnesium carbonate.

See also milk of magnesia.