- a submerged aquatic plant, Hydrilla verticillata, native to the Old World, that has become a pest weed in U.S. lakes and waterways.
- any aquatic plant of the Eurasian genus Hydrilla, growing underwater and forming large masses: used as an oxygenator in aquaria and pools. It was introduced in the S US where it has become a serious problem, choking fish and hindering navigation
