The Collaborative International Dictionary of English v.0.48:

Duckweed \Duck"weed`\, n. (Bot.)
   A genus (Lemna) of small plants, seen floating in great
   quantity on the surface of stagnant pools fresh water, and
   supposed to furnish food for ducks; -- called also
   duckmeat.
   [1913 Webster]

WordNet (r) 3.0 (2006):

duckweed
    n 1: any small or minute aquatic plant of the family Lemnaceae
         that float on or near the surface of shallow ponds