An undine is an elemental being that is associated with or the spirit of water. Undines are almost invariably depicted as being female, which is consistent with the ancient idea that water is a female element.
They are usually found in forest pools and waterfalls, and their beautiful singing voices are sometimes heard over the sound of water. The group contains many species, including oreades, nereides, limoniades, naiades, mermaids and potamides.
What undines lack, compared to humans, is a soul. Therefore, in order to achieve immortality, they must bewed a human. Although marriage with a human shortens their lives on Earth, it earns them an immortal human soul regardless. However, such a union is not without risk for the man, because if he is unfaithful he is fated to die.
The offspring of a union between an undine and a man are human with a soul, but also with some kind of aquatic characteristic, called a watermark.