The Collaborative International Dictionary of English v.0.48:

Coltsfoot \Colts"foot`\, n. (Bot.)
   A perennial herb (Tussilago Farfara), whose leaves and
   rootstock are sometimes employed in medicine.
   [1913 Webster]

   Butterbur coltsfoot (Bot.), a European plant (Petasites
      vulgaris).
      [1913 Webster]

WordNet (r) 3.0 (2006):

Tussilago farfara
    n 1: perennial herb with large rounded leaves resembling a
         colt's foot and yellow flowers appearing before the leaves
         do; native to Europe but now nearly cosmopolitan; used
         medicinally especially formerly [syn: coltsfoot,
         Tussilago farfara]