water milfoil

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of various cosmopolitan aquatic herbs of the genus Myriophyllum, having feathery, finely dissected submersed leaves and entire or toothed emersed leaves.

Century Dictionary and Cyclopedia

1. n. See milfoil.

GNU Webster's 1913

1. n. (Bot.) an aquatic herb with dissected leaves (Myriophyllum).
2. (Bot.) Any plant of the genus Myriophyllum, aquatic herbs with whorled leaves, the submersed ones pinnately parted into capillary divisions.

WordNet 3.0

1. n. an aquatic plant of the genus Myriophyllum having feathery underwater leaves and small inconspicuous flowers