The Collaborative International Dictionary of English v.0.48:

Cereal \Ce"re*al\, a. [L. Cerealis pert. to Ceres, and hence, to
   agriculture. See Ceres.]
   Of or pertaining to the grasses which are cultivated for
   their edible seeds (as wheat, maize, rice, etc.), or to their
   seeds or grain.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cereal \Ce"re*al\ n.
   Any grass cultivated for its edible grain, or the grain
   itself; -- usually in the plural.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cereal
    adj 1: made of grain or relating to grain or the plants that
           produce it; "a cereal beverage"; "cereal grasses"
    n 1: grass whose starchy grains are used as food: wheat; rice;
         rye; oats; maize; buckwheat; millet [syn: cereal, cereal
         grass]
    2: foodstuff prepared from the starchy grains of cereal grasses
       [syn: grain, food grain, cereal]
    3: a breakfast food prepared from grain