The Collaborative International Dictionary of English v.0.48:

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; such as witch grass, wild
   indigo, Amarantus albus, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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, tumbleweed, Salsola kali
       tenuifolia]
    3: bushy annual weed of central North America having greenish
       flowers and winged seeds [syn: winged pigweed,
       tumbleweed, Cycloloma atriplicifolium]
    4: bushy plant of western United States [syn: tumbleweed,
       Amaranthus albus, Amaranthus graecizans]