any of a family (Lemnaceae, order Arales) of monocotyledonous, minute flowering plants that float on ponds and sluggish streams and reproduce by a kind of budding: so called because eaten by ducks
See duckweed in American Heritage Dictionary 4
noun
Any of various small, free-floating, stemless aquatic flowering plants of the genus Lemna, growing in close, often carpetlike colonies on the surface of quiet water.