The Collaborative International Dictionary of English v.0.48:

Water cress \Wa"ter cress`\ (Bot.)
   A perennial cruciferous herb (Nasturtium officinale)
   growing usually in clear running or spring water. The leaves
   are pungent, and used for salad and as an antiscorbutic.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cress \Cress\ (kr[e^]s), n.; pl. Cresses (kr[e^]s"[e^]z). [OE.
   ces, cresse, kers, kerse, AS. cresse, cerse; akin to D. kers,
   G. kresse, Dan. karse, Sw. krasse, and possibly also to OHG.
   chresan to creep.] (Bot.)
   A plant of various species, chiefly cruciferous. The leaves
   have a moderately pungent taste, and are used as a salad and
   antiscorbutic.
   [1913 Webster]

   Note: The garden cress, called also peppergrass, is the
         Lepidium sativum; the water cress is the Nasturtium
         officinale. Various other plants are sometimes called
         cresses.
         [1913 Webster]

               To strip the brook with mantling cresses spread.
                                                  --Goldsmith.
         [1913 Webster]

   Bitter cress. See under Bitter.

   Not worth a cress, or "not worth a kers." a common old
      proverb, now turned into the meaningless "not worth a
      curse." --Skeat.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Nasturtium officinale
    n 1: perennial Eurasian cress growing chiefly in springs or
         running water having fleshy pungent leaves used in salads
         or as a potherb or garnish; introduced in North America and
         elsewhere [syn: common watercress, Rorippa nasturtium-
         aquaticum, Nasturtium officinale]