duckweed

from 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

from 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]