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]

The Collaborative International Dictionary of English v.0.48:

Calendula \Ca*len"du*la\, n. [NL., fr. L. calendae calends.]
   (Bot.)
   A genus of composite herbaceous plants. One species,
   Calendula officinalis, is the common marigold, and was
   supposed to blossom on the calends of every month, whence the
   name.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Calendula officinalis
    n 1: the common European annual marigold [syn: common
         marigold, pot marigold, ruddles, Scotch marigold,
         Calendula officinalis]