The Collaborative International Dictionary of English v.0.48:

marsh marigold \marsh mar"i*gold\ (m[aum]rsh
   m[a^]r"[i^]*g[-o]ld). (Bot.)
   A perennial plant of the genus Caltha (Caltha palustris),
   growing in wet places and bearing bright yellow flowers. In
   the United States it is used as a pot herb under the name of
   cowslip. See Cowslip.
   [1913 Webster]

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:

Cowslip \Cow"slip`\ (-sl[i^]p`), n. [AS. c[=u]slyppe,
   c[=u]sloppe, prob. orig., cow's droppings. Cf. Slop, n.]
   (Bot.)
   1. A common flower in England (Primula veris) having yellow
      blossoms and appearing in early spring. It is often
      cultivated in the United States.
      [1913 Webster]

   2. In the United States, the marsh marigold (Caltha
      palustris), appearing in wet places in early spring and
      often used as a pot herb. It is nearer to a buttercup than
      to a true cowslip. See Illust. of Marsh marigold.
      [1913 Webster]

   American cowslip (Bot.), a pretty flower of the West
      (Dodecatheon Meadia), belonging to the same order
      (Primulace[ae]) with the English cowslip.

   French cowslip (Bot.), bear's-ear (Primula Auricula).
      [1913 Webster]

WordNet (r) 3.0 (2006):

cowslip
    n 1: early spring flower common in British isles having fragrant
         yellow or sometimes purple flowers [syn: cowslip,
         paigle, Primula veris]
    2: swamp plant of Europe and North America having bright yellow
       flowers resembling buttercups [syn: marsh marigold,
       kingcup, meadow bright, May blob, cowslip, water
       dragon, Caltha palustris]