Erigeron canadensis

from WordNet (r) 3.0 (2006)

Erigeron canadensis
    n 1: 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}, {Erigeron canadensis}]

from The Collaborative International Dictionary of English v.0.48

Horseweed \Horse"weed`\, n. (Bot.)
   A composite plant ({Erigeron Canadensis}), which is a common
   weed.
   [1913 Webster]