Real Dictionary

cereal
variable noun: Cereal or breakfast cereal is a food made from grain. It is mixed with milk and eaten for breakfast.

cereal in American English
any grain used for food, as wheat, oats, or rice
of grain or the grasses producing grain
adjective: of or pertaining to grain or the plants producing it
noun: any plant of the grass family yielding an edible grain, as wheat, rye, oats, rice, or corn

cereal in British English
noun: any grass that produces an edible grain, such as oat, rye, wheat, rice, maize, sorghum, and millet