| genus hydra | (n) hydras |
| genus hydrangea | (n) type genus of Hydrangeaceae; large genus of shrubs and some trees and vines with white or pink or blue flower clusters; sometimes placed in family Saxifragaceae |
| hydrastis | (n) small genus of perennial herbs having rhizomes and palmate leaves and small solitary flowers; of northeastern United States and Japan, Syn. genus Hydrastis |