WordNet (r) 3.0 (2006):

Conyza 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]