Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

Erigeron canadensis
     n : 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: {horseweed}, {Canadian fleabane}, {fleabane},
          {Conyza canadensis}]