DUCKWEED
duckweed
(noun) any small or minute aquatic plant of the family Lemnaceae that float on or near the surface of shallow ponds
Source: WordNet® 3.1
Etymology
Noun
duckweed (countable and uncountable, plural duckweeds or duckweed)
Any of several reduced floating aquatic plants in the subfamily Lemnoideae of the family Araceae.
Source: Wiktionary
Duck"weed`, n. (Bot.)
Definition: 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: Webster’s Unabridged Dictionary 1913 Edition