The Collaborative International Dictionary of English v.0.48:

Blowball \Blow"ball`\, n.
   The downy seed head of a dandelion, which children delight to
   blow away. --B. Jonson.
   [1913 Webster] blowby

WordNet (r) 3.0 (2006):

blowball
    n 1: any of several herbs of the genus Taraxacum having long tap
         roots and deeply notched leaves and bright yellow flowers
         followed by fluffy seed balls [syn: dandelion,
         blowball]