a South American aquatic plant (Eichhornia crassipes) of the pickerelweed family, with swollen petioles that float on water and spikes of showy lavender flowers: a pest that blocks water traffic in the S U.S., esp. Florida
See water hyacinth in American Heritage Dictionary 4
noun
A tropical American herb (Eichhornia crassipes) forming dense floating masses in ponds and streams and having large bluish-purple flowers and leafstalks with greatly inflated bases.