The Collaborative International Dictionary of English v.0.48:

Pondweed \Pond"weed`\, n. (Bot.)
   Any aquatic plant of the genus Potamogeton, of which many
   species are found in ponds or slow-moving rivers.
   [1913 Webster]

   Choke pondweed, an American water weed (Anarcharis
      Canadensis, syn. Elodea Canadensis.) See Anacharis.


   Horned pondweed, the Zannichellia palustris, a slender,
      branching aquatic plant, having pointed nutlets.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pondweed
    n 1: any of several submerged or floating freshwater perennial
         aquatic weeds belonging to the family Potamogetonaceae
    2: submerged freshwater perennials [syn: Elodea, genus
       Elodea, pondweed, ditchmoss]