(a) (usually with the and capital initials) (the name of) a supernatural character in Arthurian legend, said to have given King Arthur the sword…

(See quots.)

A water-spirit.

In German folklore: a water sprite or nymph having a human torso and the tail of a fish, and able to take on a variety of physical appearances in…

A supernatural female being, imagined as inhabiting the water; a nymph. Also attributive.

= nixie, n.1

The name of: a female supernatural being or creature said to lurk beneath the surface of (esp. weed-covered) ponds, ditches, etc., waiting to…