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):

horned pondweed
    n 1: found in still or slow-moving fresh or brackish water;
         useful to oxygenate cool water ponds and aquaria [syn:
         horned pondweed, Zannichellia palustris]