The Collaborative International Dictionary of English v.0.48:

Madder \Mad"der\ (m[a^]d"d[~e]r), n. [OE. mader, AS. maedere;
   akin to Icel. ma[eth]ra.] (Bot.)
   A plant of the genus Rubia (Rubia tinctorum). The root is
   much used in dyeing red, and formerly was used in medicine.
   It is cultivated in France and Holland. See Rubiaceous.
   [1913 Webster]

   Note: Madder is sometimes used in forming pigments, as lakes,
         etc., which receive their names from their colors, such
         as madder yellow.
         [1913 Webster]

   Field madder, an annual European weed (Sherardia
      arvensis) resembling madder.

   Indian madder, the East Indian Rubia cordifolia, used in
      the East for dyeing; -- called also munjeet.

   Wild madder, Rubia peregrina of Europe; also the Galium
      Mollugo, a kind of bedstraw.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Rubia tinctorum
    n 1: Eurasian herb having small yellow flowers and red roots
         formerly an important source of the dye alizarin [syn:
         madder, Rubia tinctorum]