naiadn.[L. naias, -adis, nas, -idis, a water nymph, Gr nai:a`s, nai:`s, fr. na`ein to flow: cf. F. naade. 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.()2. (Zol.) Any species of a tribe (Naiades) of freshwater bivalves, including Unio, Anodonta, and numerous allied genera; a river mussel.()3. (Zol) One of a group of butterflies. See Nymph.()4. (Bot.) Any plant of the order Naiadace, such as eelgrass, pondweed, etc.()