Source : Webster's Revised Unabridged Dictionary (1913)

Butterweed \But"ter*weed`\, n. (Bot.)
   An annual composite plant of the Mississippi valley ({Senecio
   lobatus}).

Source : WordNet®

butterweed
     n 1: American ragwort with yellow flowers [syn: {ragwort}, {Senecio
          glabellus}]
     2: any of several yellow-flowered plants of the genus Packer;
        often placed in genus Senecio