arnica, any plant of the genus Arnica, yellow-flowered perennials of the family Asteraceae (aster family), native to north temperate and arctic regions. In North America, arnicas grow in woody areas of the plains region and the Pacific coast, northward to arctic Alaska. Medicinal preparations for the treatment of wounds and bruises are sometimes made from arnica plants, chiefly A. montana of the European Alps. Arnica is classified in the division Magnoliophyta, class Magnoliopsida, order Asterales, family Asteraceae.