The Collaborative International Dictionary of English v.0.48:

Burdock \Bur"dock\, n. [Bur + dock the plant.] (Bot.)
   A genus of coarse biennial herbs (Lappa), bearing small
   burs which adhere tenaciously to clothes, or to the fur or
   wool of animals.
   [1913 Webster]

   Note: The common burdock is the Lappa officinalis.
         [1913 Webster]

WordNet (r) 3.0 (2006):

burdock
    n 1: any of several erect biennial herbs of temperate Eurasia
         having stout taproots and producing burs [syn: burdock,
         clotbur]