The Collaborative International Dictionary of English v.0.48:

Broccoli \Broc"co*li\, n. [It. broccoli, pl. of broccolo sprout,
   cabbage sprout, dim. of brocco splinter. See Broach, n.]
   (Bot.)
   A plant of the Cabbage species (Brassica oleracea) of many
   varieties, resembling the cauliflower. The "curd," or
   flowering head, is the part used for food.
   [1913 Webster]

WordNet (r) 3.0 (2006):

broccoli
    n 1: plant with dense clusters of tight green flower buds [syn:
         broccoli, Brassica oleracea italica]
    2: branched green undeveloped flower heads