The Collaborative International Dictionary of English v.0.48:

Xanthium \Xan"thi*um\, n. [NL., fr. Gr. xa`nqion a plant used
   for dyeing the hair yellow, said to be the Xanthium
   strumarium, from xanqo`s yellow.] (Bot.)
   A genus of composite plants in which the scales of the
   involucre are united so as to form a kind of bur; cocklebur;
   clotbur.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Xanthium
    n 1: coarse herbs having small heads of greenish flowers
         followed by burrs with hooked bristles [syn: Xanthium,
         genus Xanthium]