water milfoil

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various cosmopolitan aquatic herbs of the genus Myriophyllum, having whorls of feathery submersed leaves, especially M. spicatum of Eurasia, which can form dense mats in North American rivers and lakes.

from The Century Dictionary.

• noun See milfoil.

from the GNU version of the Collaborative International Dictionary of English.

• (Bot.) Any plant of the genus Myriophyllum, aquatic herbs with whorled leaves, the submersed ones pinnately parted into capillary divisions.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an aquatic plant of the genus Myriophyllum having feathery underwater leaves and small inconspicuous flowers