horehound, aromatic Old World perennial herb ( Marrubium vulgare ) of the family Labiatae (mint family), naturalized in North America. It has woolly white foliage and tiny white clustered flowers and is called the common, or white, horehound. The dried leaves and flower tops were used in making horehound candy and remedies for coughs and colds. The black horehound and the water horehound belong to other genera of the mint family. Horehound is classified in the division Magnoliophyta, class Magnoliopsida, order Lamiales, family Labiatae.