Source : Webster's Revised Unabridged Dictionary (1913)

Undine \Un*dine"\, n. [G. undine, or F. ondin, ondine, from L.
   unda a wave, water.]
   One of a class of fabled female water spirits who might
   receive a human soul by intermarrying with a mortal.

Source : WordNet®

undine
     n : any of various water spirits