Source : Webster's Revised Unabridged Dictionary (1913)

Butterbur \But"ter*bur`\, n. (Bot.)
   A broad-leaved plant ({Petasites vulgaris}) of the Composite
   family, said to have been used in England for wrapping up
   pats of butter.

Source : WordNet®

butterbur
     n : small Eurasian herb having broad leaves and lilac-pink
         rayless flowers; found in moist areas [syn: {bog rhubarb},
          {Petasites hybridus}, {Petasites vulgaris}]