(1):

(n.) Any species of a tribe (Naiades) of freshwater bivalves, including Unio, Anodonta, and numerous allied genera; a river mussel.

(2):

(n.) Any plant of the order Naiadaceae, such as eelgrass, pondweed, etc.

(3):

(n.) One of a group of butterflies. See Nymph.

(4):

(n.) 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.