Any of various plants related to this; especially any of the aquatic or wetland plants of warm regions constituting the genus Bacopa (family Plantaginaceae), having small, often fleshy leaves and white or bluish flowers; specifically the pantropical B. monnieri.
Origin
Mid 17th century.
Definition of water hyssop in US English:
water hyssop
noun
Any of various plants related to this; especially any of the aquatic or wetland plants of warm regions constituting the genus Bacopa (family Plantaginaceae), having small, often fleshy leaves and white or bluish flowers; specifically the pantropical B. monnieri.