Undine

Undines /ˈʌnˌdiːn, ˌʌnˈdiːn/ (or ondines) are a category of elemental beings associated with water, first named in the alchemical writings of Paracelsus. Similar creatures are found in classical literature, particularly Ovid's Metamorphoses. Later writers developed the undine into a water nymph in its own right, and it continues to live in modern literature and art through such adaptations as Hans Christian Andersen's "The Little Mermaid".

Undines are almost invariably depicted as being female, and are usually found in forest pools and waterfalls. The group contains many species, including nereides, limnads, naiades and mermaids. Although resembling humans in form they lack a human soul, so to achieve immortality they must acquire one by marrying a human. Such a union is not without risk for the man, because if he is unfaithful he is fated to die.

Undine is a term that appears in the alchemical writings of Paracelsus, a Renaissance alchemist and physician. It is derived from the Latin word unda, meaning "wave", and first appears in Paracelsus' book Liber de Nymphis, sylphis, pygmaeis et salamandris et de caeteris spiritibus, published posthumously in 1658.Ondine is an alternative spelling, and has become a female given name.

...
Wikipedia