Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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