Source : Webster's Revised Unabridged Dictionary (1913)

Horseweed \Horse"weed`\, n. (Bot.)
   A composite plant ({Erigeron Canadensis}), which is a common
   weed.

Source : WordNet®

horseweed
     n 1: erect perennial strong-scented with serrate pointed leaves
          and a loose panicle of yellowish flowers; the eastern
          United States [syn: {horse balm}, {stoneroot}, {stone-root},
           {richweed}, {stone root}, {Collinsonia canadensis}]
     2: common North American weed with linear leaves and small
        discoid heads of yellowish flowers; widely naturalized
        throughout temperate regions; sometimes placed in genus
        Erigeron [syn: {Canadian fleabane}, {fleabane}, {Conyza
        canadensis}, {Erigeron canadensis}]