The Collaborative International Dictionary of English v.0.48:

Marigold \Mar"i*gold\, n. [Mary + gold.] (Bot.)
   A name for several plants with golden yellow blossoms,
   especially the Calendula officinalis (see Calendula), and
   the cultivated species of Tagetes.
   [1913 Webster]

   Note: There are several yellow-flowered plants of different
         genera bearing this name; as, the African marigold or
         French marigold of the genus Tagetes, of which
         several species and many varieties are found in
         gardens. They are mostly strong-smelling herbs from
         South America and Mexico: bur marigold, of the genus
         Bidens; corn marigold, of the genus Chrysanthemum
         (Chrysanthemum segetum, a pest in the cornfields of
         Italy); fig marigold, of the genus
         Mesembryanthemum; marsh marigold, of the genus
         Caltha (Caltha palustris), commonly known in
         America as the cowslip. See Marsh Marigold.
         [1913 Webster]

   Marigold window. (Arch.) See Rose window, under Rose.
      [1913 Webster]

WordNet (r) 3.0 (2006):

marigold
    n 1: any of various tropical American plants of the genus
         Tagetes widely cultivated for their showy yellow or orange
         flowers