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.
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               To strip the brook with mantling cresses spread.
                                                  --Goldsmith.
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   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):

cress
    n 1: any of various plants of the family Cruciferae with edible
         leaves that have a pungent taste [syn: cress, cress
         plant]
    2: pungent leaves of any of numerous cruciferous herbs