The Collaborative International Dictionary of English v.0.48:

Alyssum \A*lys"sum\ ([.a]*l[i^]s"s[u^]m), n. [NL., fr. Gr.
   'a`lysson, name of a plant, perh. fr. 'a priv. + ly`ssa
   raging madness.] (Bot.)
   A genus of cruciferous plants; madwort. The sweet alyssum
   (Alyssum maritimum), cultivated for bouquets, bears small,
   white, sweet-scented flowers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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