The Collaborative International Dictionary of English v.0.48:

Madwort \Mad"wort`\, n. (Bot.)
   A genus of cruciferous plants (Alyssum) with white or
   yellow flowers and rounded pods. Alyssum maritimum is the
   commonly cultivated sweet alyssum, a fragrant white-flowered
   annual.
   [1913 Webster]

WordNet (r) 3.0 (2006):

madwort
    n 1: any garden plant of the genus Alyssum having clusters of
         small yellow or white flowers [syn: alyssum, madwort]