ground ivy, trailing perennial herb of the genus Glechoma of the family Labiatae (mint family), closely related to catnip and naturalized from Europe. It forms a dense ground cover and spreads rapidly, thriving in cool, damp places. Its leaves were once used to brew a cough remedy; in quantity it is reputed to be poisonous to horses. Gill-over-the-ground is another of its many names. Ground ivy is classified in the division Magnoliophyta, class Magnoliopsida, order Lamiales, family Labiatae.