Undine

Undine or Ondine is an elemental spirit associated with the element of water. The term was coined by the Swiss Renaissance alchemist and physician Theophrastus von Hohenheim, also known as Paracelsus, in his book "A Book on Nymphs, Sylphs, Pygmies, and Salamanders, and on the Other Spirits," which was published posthumously in 1566.

Over time, authors and writers developed the undine into a creature similar to a female water nymph or a mermaid. In these stories men who married an Undine were doomed to die if they were ever unfaithful to her. One notable adaptation of such stories is the 1811 romantic fairytale titled "Undine" by the German novelist Friedrich Heinrich Karl de la Motte, Baron Fouqué.

Etymology
The word "undine" ultimately comes from "unda", Latin for "wave."