The Collaborative International Dictionary of English v.0.48:

Milkwort \Milk"wort`\, n. (Bot.)
   A genus of plants (Polygala) of many species. The common
   European Polygala vulgaris was supposed to have the power
   of producing a flow of milk in nurses.
   [1913 Webster]

   Note: The species of Campanula, or bellflower, are
         sometimes called milkwort, from their juice.
         [1913 Webster]

WordNet (r) 3.0 (2006):

milkwort
    n 1: any of various plants of the genus Polygala