Source : Webster's Revised Unabridged Dictionary (1913)

Tumbleweed \Tum"ble*weed`\, n. (Bot.)
   Any plant which habitually breaks away from its roots in the
   autumn, and is driven by the wind, as a light, rolling mass,
   over the fields and prairies; as witch grass, wild indigo,
   {Amarantus albus}, etc.

Source : WordNet®

tumbleweed
     n 1: any plant that breaks away from its roots in autumn and is
          driven by the wind as a light rolling mass
     2: prickly bushy Eurasian plant; a troublesome weed in central
        and western United States [syn: {Russian thistle}, {Russian
        tumbleweed}, {Russian cactus}, {Salsola kali tenuifolia}]
     3: bushy annual weed of central North America having greenish
        flowers and winged seeds [syn: {winged pigweed}, {Cycloloma
        atriplicifolium}]
     4: bushy plant of western United States [syn: {Amaranthus albus},
         {Amaranthus graecizans}]