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.-any species of a tribe (naiades) of freshwater bivalves, including unio, anodonta, and numerous allied genera; a river mussel.-one of a group of butterflies. see nymph.-any plant of the order naiadaceae, such as eelgrass, pondweed, etc.