The Collaborative International Dictionary of English v.0.48:

Water hyacinth \Wa"ter hy"a*cinth\ (Bot.)
   Either of several tropical aquatic plants of the genus
   Eichhornia, related to the pickerel weed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water hyacinth
    n 1: a tropical floating aquatic plant having spikes of large
         blue flowers; troublesome in clogging waterways especially
         in southern United States [syn: water hyacinth, water
         orchid, Eichhornia crassipes, Eichhornia spesiosa]