The Collaborative International Dictionary of English v.0.48:

nymph \nymph\ (n[i^]mf), n. [L. nympha nymph, bride, young
   woman, Gr. ny`mfh: cf. F. nymphe. Cf. Nuptial.]
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   1. (Class. Myth.) A goddess of the mountains, forests,
      meadows, or waters.
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            Where were ye, nymphs, when the remorseless deep
            Closed o'er the head of your loved Lycidas?
                                                  --Milton.
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   2. Hence: A lovely young girl; a maiden; a damsel.
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            Nymph, in thy orisons
            Be all my sins remembered.            --Shak.
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   3. (Zool.) The pupa of an insect; a chrysalis.
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   4. (Zool.) Any one of a subfamily (Najades) of butterflies
      including the purples, the fritillaries, the peacock
      butterfly, etc.; -- called also naiad.
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The Collaborative International Dictionary of English v.0.48:

Naiad \Na"iad\ (n[=a]"y[a^]d; 277), n. [L. naias, -adis,
   na["i]s, -idis, a water nymph, Gr nai:a`s, nai:`s, fr. na`ein
   to flow: cf. F. na["i]ade. Cf. Naid.]
   1. (Myth.) A water nymph; one of the lower female divinities,
      fabled to preside over some body of fresh water, as a
      lake, river, brook, or fountain.
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   2. (Zool.) Any species of a tribe (Naiades) of freshwater
      bivalves, including Unio, Anodonta, and numerous
      allied genera; a river mussel.
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   3. (Zool) One of a group of butterflies. See Nymph.
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   4. (Bot.) Any plant of the order Naiadaceae, such as
      eelgrass, pondweed, etc.
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WordNet (r) 3.0 (2006):

naiad
    n 1: submerged aquatic plant having narrow leaves and small
         flowers; of fresh or brackish water [syn: naiad, water
         nymph]
    2: (Greek mythology) a nymph of lakes and springs and rivers and
       fountains

Moby Thesaurus II by Grady Ward, 1.0:

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