naiad

n.

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