The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undine
    n 1: any of various female water spirits

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "undine":
   Davy, Davy Jones, Neptune, Nereid, Oceanid, Oceanus, Poseidon,
   Thetis, Triton, elemental, elemental spirit, fresh-water nymph,
   gnome, kelpie, limniad, man fish, mermaid, merman, naiad, nix,
   nixie, ocean nymph, salamander, sea nymph, sea-maid, sea-maiden,
   siren, sylph, water god, water spirit, water sprite