The Collaborative International Dictionary of English v.0.48:

Laburnum \La*bur"num\, n. [L.] (Bot.)
   A small leguminous tree (Cytisus Laburnum), native of the
   Alps. The plant is reputed to be poisonous, esp. the bark and
   seeds. It has handsome racemes of yellow blossoms.
   [1913 Webster]

   Note: Scotch laburnum (Cytisus alpinus) is similar, but has
         smooth leaves; purple laburnum is Cytisus purpureus.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Laburnum
    n 1: flowering shrubs or trees having bright yellow flowers; all
         parts of the plant are poisonous [syn: Laburnum, genus
         Laburnum]