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

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "cereal":
   aftergrass, bamboo, botanic, breakfast food, bulbous, cane,
   cereal plant, corn, cornflakes, dry cereal, farina, farinaceous,
   farinaceous plant, fog, forage grass, fruitlike, fruity, frumenty,
   grain, graminaceous plant, grass, grits, gruel, hasty pudding,
   herbaceous, herbal, herbose, herbous, herby, hominy grits,
   hot cereal, kasha, lawn grass, leguminose, leguminous, loblolly,
   millet, mush, oatmeal, ornamental grass, plantlike, porridge,
   puffed rice, puffed wheat, radicated, radiciform, radicular, reed,
   rhizoid, rolled oats, rootlike, tuberous, vegetable, vegetal,
   vegetarian, vegetational, vegetative, weedy, wheatflakes