The Collaborative International Dictionary of English v.0.48:

Nasturtium \Nas*tur"tium\, n. [L. nasturtium, for nasitortium,
   fr. nasus nose + torquere, tortum, to twist, torture, in
   allusion to the causing one to make a wry face by its pungent
   taste. See Nose of the face, and Torture.]
   1. (Bot.) A genus of cruciferous plants, having white or
      yellowish flowers, including several species of cress.
      They are found chiefly in wet or damp grounds, and have a
      pungent biting taste.
      [1913 Webster]

   2. (Bot.) Any plant of the genus Tropaeolum, geraniaceous
      herbs, having mostly climbing stems, peltate leaves, and
      spurred flowers, and including the common Indian cress
      (Tropaeolum majus), the canary-bird flower (Tropaeolum
      peregrinum), and about thirty more species, all natives
      of South America. The whole plant has a warm pungent
      flavor, and the fleshy fruits are used as a substitute for
      capers, while the leaves and flowers are sometimes used in
      salads.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nasturtium
    n 1: any tropical American plant of the genus Tropaeolum having
         pungent juice and long-spurred yellow to red flowers
    2: aquatic herbs [syn: Nasturtium, genus Nasturtium]
    3: flowers and seeds and leaves all used as flavorings