horseradish - a cruciferous plant (Cochlearia Armoracia), with white flowers and broad rough leaves, a native of middle Europe and western Asia, commonly cultivated for its root, horseradish - hardy perennial plant of the mustard family (Brassicaceae, or Cruciferae) or its hotly pungent, fleshy root, used as a condiment or table relish mainly in the form of a sauce to enhance seafoods and meats; the root is traditionally considered medicinal. Native to Mediterranean lands, horseradish is now grown throughout the temperate zones. In many cool, moist areas it has become a troublesome weed. Large, coarse, glossy-green basal leaves arise on long stems from the crown atop the large white root. Small white flowers are borne in terminal or axillary racemes; small oblong pods are tipped by a short, persistent style.