Historically, Hydromel was a beverage known in classical Greece and Rome. It appears to have simply been a term used for mead, referring to the addition of water ("Hydro") to honey ("Mel"). In some cases, hydromel may have referred to a strong mead that was diluted with water before drinking. When salt water was used to dilute the mead, the resulting beverage was known as a thalassiomel.
Among modern mead makers, "hydromel" is generally used to describe either a watered-down mead or a mead brewed initially to a low gravity. Other terms for a low-gravity mead include Small Mead.