any of various plants (as genera Onopordum, Cirsium, and Cnicus) of the composite family, with prickly leaves and heads of white, purple, pink, or yellow flowers; esp., the (O. acanthium) with white down and lavender flowers
See thistle in American Heritage Dictionary 4
(thĭsˈəl)
noun
Any of numerous weedy plants, chiefly of the genera Cirsium, Carduus, or Onopordum of the composite family, having prickly leaves and variously colored flower heads surrounded by prickly bracts.