'Naiad' definitions:

Definition of 'naiad'

noun

Submerged aquatic plant having narrow leaves and small flowers; of fresh or brackish water [syn: naiad, water nymph]

noun

(Greek mythology) a nymph of lakes and springs and rivers and fountains

From: GCIDE

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. [1913 Webster]
2. (Zool.) Any species of a tribe (Naiades) of freshwater bivalves, including Unio, Anodonta, and numerous allied genera; a river mussel. [1913 Webster]
3. (Zool) One of a group of butterflies. See Nymph. [1913 Webster]
4. (Bot.) Any plant of the order Naiadaceae, such as eelgrass, pondweed, etc. [1913 Webster]

From: GCIDE

nymph \nymph\ (n[i^]mf), n. [L. nympha nymph, bride, young woman, Gr. ny`mfh: cf. F. nymphe. Cf. Nuptial.] [1913 Webster]
1. (Class. Myth.) A goddess of the mountains, forests, meadows, or waters. [1913 Webster]
Where were ye, nymphs, when the remorseless deep Closed o'er the head of your loved Lycidas? --Milton. [1913 Webster]
2. Hence: A lovely young girl; a maiden; a damsel. [1913 Webster]
Nymph, in thy orisons Be all my sins remembered. --Shak. [1913 Webster]
3. (Zool.) The pupa of an insect; a chrysalis. [1913 Webster]
4. (Zool.) Any one of a subfamily (Najades) of butterflies including the purples, the fritillaries, the peacock butterfly, etc.; -- called also naiad. [1913 Webster]