waterwort

from The Century Dictionary.

• noun A plant of the genus Elatine, or more broadly of the order Elatinaceæ, primarily E. Hydropiper of the Old World.
• noun The plant Philydrum lanuginosum, or (Lindley) any plant of the order Philydraceæ.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) Any plant of the natural order Elatineæ, consisting of two genera (Elatine, and Bergia), mostly small annual herbs growing in the edges of ponds. Some have a peppery or acrid taste.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of the family Elatinaceae of aquatic plants.